Every 25% of the Deconstructor's health activates phase 2 of the fight (75%, 50%, 25%), which exposes his heart.
Any damage you do to the heart is passed onto Deconstructor, so it is imperative that you do as much DPS to the heart as possible without killing it (unless you want to activate hard mode). We only had DPS to get the heart down to 65% health, and I think that was largely our problem last night. Some of that might be attributed to deaths from Light Bombs and Gravity Bombs either because people weren't paying attention or because healers couldn't heal them fast enough. Or because too much DPS was pulled off the heart to control the adds during phase 2.
Phase 2: Exposed Heart and Adds (from WoWWiki)
Whenever the boss gets below a 25% threshold (at 75%, 50% and 25%), it switches for 20 seconds to Phase 2. During this time, the boss itself cannot be targeted, but it exposes its heart, takes double damage, and adds are summoned. Three types of adds spawn from the scrap heaps in the four corners of the room:
- XS-013 Scrapbot: heals XT-002 for 60k each if they reach it.
- XE-321 Boombot: explode for ~16k fire damage upon getting killed or upon reaching XT-002, dealing damage to any players or NPCs in range.
- XM-024 Pummeler: these deal low single-target damage.
The Scrapbots need to be AoE'd down and must not reach the boss. They are susceptible to most snares (stun, frost nova, knockback, etc.). The Boombots should be single targeted and nuked by ranged. It is best if they explode when surrounded by scrapbots or near the Heart. The Pummelers should which should simply be off-tanked and kited during the whole fight around the room. No damage dealer should ever target a Pummeler.
Scrapbots spawn in ~7-10 groups of 5. The groups can appear simultaneously and at varying times. Have people keep an eye out and call out where and when they are coming. When scrapbots get closer to the raid group, they need to be AOE'd down immediately (a Holy Priest using Mind Sear can help reduce the number of DPS wasted on scrapbots). Boombots need to be DPS'ed when they are near the Heart or scrapbots (or when not close to players).
When the heart goes back up into the boss and XT becomes targettable again, unkilled adds stay alive.
The Heart has 10 M hitpoints (HC) or 1.5 M in normal mode. Since the heart takes double damage, and all damage it takes is passed through to the boss, every damage dealer should try to concentrate on the Heart as much as possible. Melee should stay on the heart exclusively during the whole phase. Even healers can deal some damage in this phase, Mana permitting (else healers can use this phase to regenerate a little). While it's desirable to deal as much damage to the heart as possible, destroying it invokes hard mode, pay attention that this doesn't happen by accident.
In order to maximize the number of players who profits from DPS cooldowns it's better to use it during the first adds phase. If there are not many deaths it's more effective to use Heroism or Bloodlust in the third adds phase, because in that phase usually 20% boss health is reached, and players with Execute or Decimate profit from the last few seconds.
If we could have put the heart down into the 50's and controlled the adds a bit better I think we could have downed him in only a few attempts. Fortunately we're still running Naax-25 and OS-25 on Wednesday nights so the newer people in the raid can get better gear. That should improve things in the coming weeks, but it won't help those who are having problems performing exterior activities like staying out of the fire on Razorscale or getting out away from people when they have light bomb or gravity bomb.
I've now run Ulduar a few times and I am even more of the opinion that Blizzard did a pretty good job on this instance. While we fairly regular posts on the official forums complaining about how "easy" Ulduar is, I think it's going to be quite a challenge for the majority of players for some time to come. It's definitely a step up from Naaxramus and I'm personally quite happy with the greater difficulty level.
While I haven't yet seen most of the instance I am also pleased with the obvious attempt Blizzard made to create new and unique boss fights. Especially Flame Leviathan, which is a much better vehicle fight implementation that the Eye of Eternity in my opinion.