The Construct Quarter probably one of the most lethal places in a raid zone that do not involve a boss, possibly because of the large variety of obstacles (not just mobs) that the raid has to contend with.
The way to Patchwerk has the most trash you'll see in the quarter, and nearly all of it can be AoE-ed, but the abominations have an AoE stun so you'll want to stand away from them as a healer. In the first big room there is also large slime. It's not that special on its own, but after it's killed, there is the ongoing possibility of another one being dropped on the raid whenever Grobbulus pats on the ledge above the room. Just keep alert while moving on to the slime pack in the next room.
Said slime pack needs to be AoE-ed down and avoided by melee since they will probably kill whoever they touch. Sometimes an overzealous AoE might grab aggro from all of them and not get out of the way in time, in which case a quickly timed Hand of Protection could just save some clothie's life. If you're feeling cocky and no one needs healing, you can run into the middle of the slimes while they're aggroed on someone else and drop a Consecrate.
From here you'll want to watch out for the river of green slime. It reduces your health and mana to about 30% when you step out of it. You can stand it in and take minimal on-going damage if it's not a good time for you to lose so much health and mana (say the raid's fighting trash), but if you can manage to jump over it, it's better to do so.
After Patchwerk comes "frogger" which is a pack of non-targetable slimes which will instantly kill anyone who doesn't have something like Bone Shield or Divine Protection up. If you want to be nice, you can bubble up and kill a few slimes by having them splat on your bubble to clear a path for other people. Usually somebody ends up dying here, so be ready to rez and rebuff them.
The only other trash of note are the giants after Gluth. They have an AoE knockback which is a real pain. You will probably only have to kill two or three of them, and it's easiest if the tanks drag them back into Gluth's room so there's no chance of anyone being flung into adds. As a healer they're annoying because they seem to like punting me in the middle of a heal and they hit pretty hard. Beacon is definitely helpful while fighting these guys since you will probably need to heal multiple people.
Patchwerk is commonly considered to be the gear check of Naxx. He is a simple fight, but tests the combined gear of your tank and off-tank (can they survive the hits), your dps (can the raid kill him before his enrage timer), and your heals (can the healers keep up the MT and OT given the damage they're taking).
Patchwerk has no gimmicks other than his Hateful Strike ability. Once the main tank has aggro he can tank him wherever he pleases, though if the OT is having trouble being hit by the Hateful Strike, then it may be helpful to tank Patchwerk near the green slime so the melee dps can step in it.
Now why would we want the melee to step in the green slime that lowers their health and mana to a third of its normal value?
The Hateful Strike hits the melee with the highest health that is not the tank. Hateful Strike hitting the dps is bad, and stepping in and out of the slime will prevent the melee from being hit by it because their health will clearly be lower than the OT's (just make sure you don't heal them!).
Aside from the Hateful Strike mechanic, Patchwerk is tank and spank. The Hateful Strike soaker will take a lot of damage, but it's mitigated by defense, dodge, and other useful tanking stats (which is why the soaker should be an actual tank). This is a very classy fight for Beacon because only two targets should be taking any damage, so we can heal one and have the heal duplicated on the other.
Generally the Hateful Strike tank will end up taking more damage that the MT, so if you're the only holy paladin, it may be a good idea to Beacon the MT and concentrate on healing the OT (short-changing the MT due to an overheal is less dangerous than short-changing the OT).
If for some reason the MT is taking more damage (perhaps because the raid put the better geared tank on Hateful Strike soaking, which is a viable strategy), then feel free to switch the Beacons. Nutshell is the tank who is taking less damage should be the Beacon in order to maximize the heals you'll get out of it.
If there is any fight where you are likely to run out of mana due to being a newly geared holy paladin in Naxx, this is it. If you can last through the constant healing of Patchwerk you should be able to heal anything else in the instance without going oom.
Grobbulus can be a frustrating fight because one person who doesn't pay attention could potentially ruin it for the rest of the raid, so raid members have to be alert. This is a two tank fight, with one tank handling Grobbulus himself, and the other handling the slimes that Grobbulus will spawn throught the fight.
This is potentially a very mobile fight, which means it works against the paladin healing style, but keeping up Judgements of the Pure and making liberal use of Holy Shock should go a long way to speeding up your heals.
Generally the MT will tank Grobbulus in a square around the edge of the room. Melee has to be very careful to stay to the side and slightly behind the boss. In back is bad, because Grobbulus will periodically drop a Poison Cloud under him and the tank will move him to get out of it. Obviously, being directly behind Grobbulus would mean you're standing in a Poison Cloud, and that is BAD.
Being in front of Grobbulus is also bad because he does a Slime Spray that will create a slime add for each person it hits. The MT getting hit is unavoidable, but no one else should be hit by it. Once the slime spawns it'll be up to the OT to pull it away. We generally assign a couple of dps to help kill the slimes.
If dealing with the boss-generated Poison Clouds was not enough, we also have to deal with his Mutating Injection attack, which will randomly hit one person in the raid at regular intervals of about 20 seconds.
Mutating Injection lasts for 10 seconds and when it expires it'll drop a poison cloud wherever the injected person was standing. Though this is a disease and can be cleansed by paladins, DON'T DO IT! If the disease is cleansed then it has the same effect as if it naturally ran out, leaving the raid with a poison cloud wherever that person was standing.
If you leave the injection ticking, then the infected person will have time to run away and safely "explode" away from the rest of the raid.
We try to have people explode behind the poison cloud trail that Grobbulus leaves behind, but there are some alcoves along a couple of the walls that people can safely explode in without getting in the way of the tank's path. You never want to force the tank to path through a poison cloud, and you'll want the middle of the room to be clear for the majority of the raid to stand in.
Some paladins may be talented for Pure of Heart, because they share a spec for PvE and PvP, in which case they must be extra cautious on Grobbulus. This will shorten the Mutating Injection duration to 5 seconds, leaving even less time to get out of the way and safely explode where no one else will be hurt.
Because I may end up running around a lot in this fight, either because I'm injected or because I'm following the tank, I find it easiest to Beacon the main tank. Sometimes I'll be out of range of the tank, because I'm across the room exploding, but I'll still be able to heal him by healing someone else who is closer.
Unlike the other quarters, the third boss is not the final one. Gluth is an odd sort of animal (pun not intended) in that his drops are all random picks off other bosses. He needs two tanks minimum, and a third sort of off-tank that can be a dps class or even a holy paladin (if the healing can be spared).
The two main tanks will generally tank Gluth on the far side of the room from where you enter because he needs to be kept away from his zombie chow (and the pipe they come from is near the entrance). Every 10-15 seconds Gluth will inflict a stacking Mortal Wound on his current target, reducing the healing done to them. Because that would eventually make a single tank unhealable, Gluth must be taunted by a second tank, giving the first tank the 15 seconds needed for the debuff to wear off. Our tanks tend to switch at around 4-5 stacks if similarly geared. If a newer tank is along for the ride they will only keep Gluth long enough for the debuff to wear off the more experienced one.
Gluth has a Enrage ability he will occasionally use that is different from the "you've taken too long to kill the boss" enrage. When he uses it, he'll grow big and red and his attack speed will increase. This ability can be canceled by a hunter's Tranq Shot or a rogue's Anesthetic Poison, but if neither are available, it's not that difficult to heal through.
While Gluth is being kept occupied, there will be a slow stream of zombies spawning in the back of the room. These zombies inflict a stacking Infected Wound every time they hit someone, so they must be kited. We've had good success with DKs, ret pallies, and proper tanks keeping them occupied, but in 10-man it's possible that there will not be a suitable tank/dps available in which case a holy paladin can work as well.
When kiting the zombies you need to keep all their aggro on you so that they aren't attacking the other healers or the dps in the raid, but you also want to stay at range as much as possible to avoid the Infected Wounds. You can do this with Holy Wrath (to stun them while you get away) and dropping Consecrates for them to walk through. You can also use Exorcism or Judgements to try pulling any lone stragglers. Righteous Fury should be up for the extra threat.
The kiter, whether it is you or someone else, should have some support. A healer should remain in range of the kiter, shamans should drop Earthbind totems, and hunters should drop Frost Trap to make it easier to get the zombies all piled into one location for easy kiting. If you are healing the zombie kiter and the kiter is in the far back of the room, it is very possible you will not be able to reach the Gluth tanks, in which case just make sure the raid leader knows. If the kiter is particularly good it may be possible to stay primarily on the Gluth tanks and then run back and spot heal the kiter as needed.
Even with slowing effects, the zombie kiter is likely to end up taking some damage and get severals stacks of Infected Wounds, particularly if it's their first time, so be willing to help out with a Holy Wrath if the kiter is getting mobbed. It's not healing per se, but it might prevent them from being overwhelmed, and good stun will give the kiter a chance to get some distance.
About once every two minutes Gluth will do a horrible attack called Decimate which will reduce everyone's health to 5%. This includes the tanks! When Decimate happens you have just a few seconds to heal up the current tank before Gluth kills him.
If you have a good boss mod up, you probably can wind up a Holy Light just as the timer bar counts down to Decimate, but even if the mod is not that accurate or you don't have one, there should be enough time to cast a HL. If you're really paranoid about your reaction time, use the Divine Favor + Holy Shock combo and follow up with HL.
Beacon is really nice here because you can put it on one of the tanks, heal the other one, and both tanks should be sitting pretty shortly after the Decimate occurs.
During Decimate, all the dps classes will be busy AoE-ing down the zombie chow before they reach Gluth, because each zombie devoured will heal Gluth by 5%. I never find any time to dps at this stage because I'm busy spamming recovery heals, but if for some reason healing is really strong and dps is weak it might be possible, even desireable, to drop a Consecrate/Holy Wrath as the zombies walk by.
Because Gluth can heal by eating the zombie chow, it's entirely possible that a novice raid group will end up hitting the 8-minute enrage timer, particlarly if they do not have a good kiter to group up the zombies or the dps is not good about killing the zombies after the Decimate.
Thaddius is probably the one fight that is leaps and bounds easier on 10-man than 25-man, but that doesn't mean he's a total cakewalk for the first-timer. He has familiar tactics to some of us, largely because Mechano-Lord Capacitus from Heroic Mechanar used his mechanics.
When the fight begins, the raid will be split in half, with one tank and assorted dps/healer(s) going up one ramp, and the other tank and assorted dps/healer(s) going up the other.
At the top of the two ramps are two mini-bosses, Stalagg and Feugen. Both of these bosses must die within a few seconds of each other or BAD THINGS HAPPEN. WoWWiki says they will resurrect each other at full health, but in my experience we just got electric shocked to death as if we'd pulled them too far away from their Tesla Coils. Either way, you'll want to kill them at the same time.
Everyone should be standing on the platform of their respective mini-boss, even if they can cast from the top of the ramp. Sometimes Stalagg and Feugen seem to want to pull away from their Tesla Coils even though the tanks have aggro on them, and we've only had this issue when people are still standing on the ramp, so my raid group has everyone stand well up on the platforms.
Beacon is pretty useless for this part of the fight because every 30 seconds there is a Magnetic Pull that will cause the tanks to switch places, and the platforms are far enough apart it is not possible to heal across them. Fortunately Stalagg and Feugen don't hit very hard, so it's fine healing without it.
The most important thing is just to keep the tanks alive. Ideally you want your tank to leave you topped up so he's at full health or close to it when he arrives on the other side. It can be difficult for a healer to save a badly injured tank that is just coming into range if they're not prepared for it. You can still heal the tank leaving you for a split second after he's thrown in the air, so keep casting a heal if you're in the middle of one, or use a Holy Shock if you're not. You might still catch him as he's leaving.
Once both Stalagg and Feugen are dead, the raid has about 15 seconds to jump off their platforms and on to Thaddius's before Thaddius activates. If you fail to make it, Thaddius will activate before you're able to run back and try again. I try to jump from the lip of the platform. If I'm that far along I find I'll make it. Some classes can help people across with Slowfall, Levitate, Aspect of the Pack, etc. but it's a good idea to be able to do this on your own because someday you'll probably have to.
If you fall, you'll land in that ugly green slime that will reduce your health and mana every time you step out of it, and you'll have to run back towards the entrance of the room to get out of the slime. From there you can run back up the ramps to the mini-boss platforms and try again.
If you fall more than once (hopefully you won't, but it happens), the raid will probably have been assigned positive and negative charges in which case you must be very careful when you land on Thaddius's platform and stay away from the rest if the raid until you are also assigned a charge.
It is possible for you to be assigned a charge while in the slime pit, and if someone else is in the slime pit with you, and they end up with an opposite charge, you can zap each other. In one 25-man raid I stepped out of the slime (causing my health to be low) and a split second later I got zapped by my guildie who also missed the jump and I died. You don't want to end up dead far, far away from the rest of the raid.
Shortly after Thaddius comes to life he will cast Polarity Shift and give everyone within range a positive or a negative charge. Polarity Shift is a 3 second cast, so even if you don't have a mod, you can see the shift coming if you have enemy cast bars turned on.
The raid will want to have all positive charges standing on one side of Thaddius and all negatives on the other. Ranged and healers should stack up close to the similarly charged melee because damage will increase by 10% for each stacking charge.
Every 30 seconds after the first charge, Thaddius will recast Polarity Shift. It is possible that your charge will not change, so it's important to watch your debuff. If you're within 10 yards of an opposite charge you'll do 3500 damage a second to that person (and they'll be doing the same to you!). The biggest issue in the Thaddius fight is losing too many people due to crossed charges. While it's okay to lose a couple people in 10-man, this becomes significantly more difficult in 25-man, which relies heavily on having enough stacked charges.
People will probably not die if they cross charges with one, or even two other players, as long as they are at full health. Most deaths occur when a player is too slow to move to the opposite side and only the slowpoke will end up getting killed. If they're lucky, they'll have a smidgen of health left so they can get healed, but if they die, it's most likely not your fault.
Sometimes people get Thaddius's Chain Lightning confused with the crossed charges though. This attack will hit five random people in the raid for 3600 to 4400 damage and could possible finish off a player who's just been weakened by crossing charges. He casts it every 15 seconds, so if the damage is coming pretty steady (once a second) it's probably a crossed charge hitting someone. If it's a sudden damage spike to multiple people it's probably Chain Lightning.
When fighting Thaddius, your priority should be moving whenever your charge changes. Even if you're in the middle of a heal, break it off and move. Holy Shock or even Lay on Hands if it matters so much that you need to get a heal off, but if you don't get yourself on the proper side there's a good chance that you're either going to end up dead, or you're going to be healing more damage than that one heal would have fixed.
There's only one tank needed for this part of the fight, so I generally Beacon him and then raid heal. I didn't find damage to be that heavy so I usually cast Flash of Light, with a few Holy Lights tossed in when needed.
Even after Thaddius is dead the raid still needs to stay split up because the charges take some time to wear off. If this is your raid's first time fighting him, there are probably one or two dead people. If you rez them before the charges wear off, ask them to not accept until the charges are gone. This is because uncharged people count as a third charge and will damage both positives and negatives.
And on a related note, if you are soulstoned or battle rezzed, you must be extremely careful should you choose to accept! My guild errs on the safe side and 90% of the time we will tell people not to rez in the middle of combat. This is because the newly rezzed person will not have a charge, and they will rez in the middle of people (either where they died or on top of the druid).
But there is a brief grace period after Thaddius casts Polarity Shift during which opposite charges will not hurt each other (so the people who need to change sides have time to do so). If you are daring and your raid is cool with it, it is possible to rez after a Polarity Shift and run away from everybody else so you are as far away from Thaddius as possible. At this distance you will not hurt anyone because you do not have a charge. You'll still be able to heal them though, and you'll get a new charge with the next Polarity Shift so it will be possible to reintegrate yourself with the rest of the group.
If you screw up by rezzing too early or too late, at best you'll end up dead (because your health will be too low to survive crossing charges) and at worst you'll take other people down with you, so rez at your discretion. I wouldn't advise doing it at all if you have latency issues.
Healing 10-man Naxx Overview - Part 5 - Sapphiron and Kel'Thuzad
Healing 10-man Naxx Overview - Part 1 - Arachnid Quarter
Healing 10-man Naxx Overview - Part 2 - Plague Quarter
Healing 10-man Naxx Overview - Part 3 - Military Quarter