My guild downed 10-man Ignis in Ulduar last night, and it was a fight that had bothered us both in 25-man and in our previous 10-man attempt on Saturday. But we finally nailed it, and it only took us two shots.
There are two ways to beat Ignis. One is to kite his Iron Construct adds the entire fight and just burn him down, but doing this results in him getting a stacking buff for each construct up that increases his damage by 15%. Once there are enough constructs up this will eventually one-shot the tank and chances are most guilds aren't going to have the dps to do this. We tried, getting him down to 30% on our best attempt, but we couldn't kill him fast enough for this method to work.
So we opted for the other method; rooting the constructs in the scorched ground created by Ignis's Scorch attack, dragging them to the water where they can turn Brittle, and then shattering them to remove the buff.
The first thing we changed from our previous 10- and 25-man attempts was where we tanked Ignis. Out tank would now move him in what amounted to a square marked by the edges of the two pools of water (each end of a pool could be considered one corner of the square). It was important that Ignis would be tanked near, but not too close to the water, where the Scorches would be put out. This would place the Scorches close to the pools of water for minimal kiting distance. This was a vast improvement over trying to keep Ignis more or less where he starts the fight, dragging the constructs over him, and then dragging them away to the water after they become molten.
Secondly, at least on 10-man, add duty can be handled entirely with a single off-tank and a moonkin, leaving the rest of the dps free to focus on the boss. In fact, moonkin possess qualities that make them ideal for add duty.
The reason for this is because of our Entangling Roots spell, which is perfect for rooting a single construct in the Scorch. Our roots are easily recasted should they break early, and because they have a chance break on damage, it's possible to prematurely break them once the construct is molten or if the Scorch has faded away and construct needs to be moved.
Additionally, our Starfires will hit for the 5k minimum to shatter a brittle construct without requiring a crit. (Though the constructs have an additional 50% chance to be crit when Brittle, why take the chance?)
I love taking on special assignments, and while I was originally concerned with being both rooter and nuker, given how painful it was on 25-man with the old strategy, it actually turned out pretty well.
Here's what we did:
At the start of the fight there are no adds, so I would cast Faerie Fire and my dots on the boss and proceed to nuke. I'd raid marked the OT so he would be easy to see, and now that the mini-map now shows raid members as colored dots based on class, it's easier to find his position that ever. Our OT was a warrior, and the only warrior in the raid so it was easy to look for the brown dot on my mini-map.
As soon as the first add would spawn I'd see our OT grab it and I'd watch him drag it over to the first Scorch that Ignis left behind. At first he would call for a root when he'd positioned the construct, but by our second attempt of the night the two of us had gotten into such good sync that he could just stun the construct and leave it in the Scorch (so he could pick up another add) and know that I'd have it rooted before it had a chance to recover.
It takes 20 stacks of the Heat debuff for the construct to turn Molten, which turns out to be about 20 seconds. Entangling Roots lasts for 27 seconds, so a single root should be enough unless it happens to break early. Mine never did unless I was actually trying to break it with additional dots and nukes.
Once the construct is Molten it starts doing a mild AoE around it and aggro is reset. Because the last thing the construct will "remember" is the damage from my roots, it would immediately go for me. Generally I will dot the construct, nuke if there's time, just to make sure my threat is above that of a healer, and the construct will go straight for me after breaking free.
I always made sure to stand in the water (with a good swath of water between me and the construct) while dotting up a molten construct, so that way once it broke free it would make a beeline for me and land in the water before it could reach me.
Once the molten construct lands in the water it will turn Brittle, almost immediately, and from then on there's about 10-15 seconds during which it will be stationary and inactive. It's enough time that it's possible to stop casting to avoid the silence effect from the boss's Flame Jets attack as well as land after having been knocked up into the air. I would always strafe away from the construct before nuking it with a Starfire (for that guaranteed kill) so I wouldn't get hit by the construct's Shatter when it explodes. Shatter will kill or nearly kill just about any non-tank and this fight is intensive enough of the healers without making it worse for them.
At this point our OT would be positioning the next construct (who would have activated by now) in another Scorch so we could repeat the process. The entire fight becomes a dance between me and the OT trading off constructs in a cycle of positioning them, rooting them, kiting them, and then nuking them.
On a successful attempt there should never be more than two constructs up at once, three on the off chance something bad happens to the kiter (getting repeatedly stuffed in the Slag Pot will do it).
As the kiter, I was sure to call out over vent whenever I ended up in the Slag Pot. This was important because the OT needs to know that I will not be able to root anything for him. If roots should happen to break, he'd need to pick up the adds, and if he knows I'm unavailable to root then he won't try position anything in the Scorch where he'd risk damaging himself.
Also, at least for 10-man, I was the only capable rooter/ranged nuker so if I died it would be a wipe. The healers would need to know if I was in danger, no different from a tank.
It's possible to heal while in the Slag Pot, and I saved my health potion for it as well. However, it is not possible to root while in the pot, even if the target is otherwise in range, so my focus was on keeping myself alive.
For my trouble I escaped a moonkin Hot Pocket and earned The Siege of Ulduar.
Side note: I'm not part of what would normally be considered a hardcore progression guild, but we were apparently the first guild on our server to down Ignis on any difficulty! It's nice to have gotten first at something, especially on the boss that most guilds have been skipping in their rush to enter Ulduar proper.