This boss cannot be tanked until his health reaches 30%, so both tanks will be tanking adds in Phase 1, and it's recommended to use two tanks for them. FYI Healers - the guide says that tank damage is low but raid damage high for this fight.
It is recommended that the melee dps the boss's leg at all times, you move him by damaging his leg, so the guide recommends that you all dps the left leg and move him in circles around the room (he's going to move left if you dps left leg, right if you dps right leg). Sounds easy, but you want to also kite him into some things which spawn on the floor and must keep him away from other things (or it's a wipe). This, evidently, requires a lot of communication on vent, and it's suggested to use a melee dps to call out instructions to the other dps during the fight. Ranged dps can stand in the middle with the healers and kill the adds.
Don't let Rhyolith reach the outer circle of the room, or it's a wipe. Don't let him run in a straight line, keep him circling. Volcanoes will spawn, try to kite him through them, but watch out for eruptions which must be avoided. After the volcanoes erupt they become craters. When he steps into a crater, it reduces his stack of Obsidian Armor. He starts the fight with 80 stacks Obsidian Armor, which reduce his damage taken by 80%. Each crater will take away a stack and his damage reduction by 1%.
Rhyolith casts concussive stomp, everyone should jump to avoid being damaged by it.
Rhyolith casts Thermal Ignition which will spawn 5 small adds which should be tanked by 1 tank. They must die within 30 seconds or they will explode and kill people. They deal more damage the longer they are alive, so they should be a dps priority for the ranged.
Rhyolith also summons a big add called the spark of Rhyolith. It hits really hard and does loads of aoe, they say this spark causes most wipes. The add should be picked up quickly, misdirects are really helpful and tanked away from the raid. Always move away from this add unless you are the main tank.
Stop dps on Rhyolith at 32% and allow his molten armor stacks to drop off, then all dps focus on him and bring him to 30%. Never let him go into phase 2 with molten armor or it's a raid wipe. Melee dps should join all the others in the center for big aoe heals.
At 30% hp, the boss turns into a fire golem and does loads of aoe damage, burn all aoe heal cooldowns to survive the transition. At this point the boss will be tankable. Pick him up, melee return to the boss, blow heroism and spank him til he's dead.
Evidently it's quite hard to get used to moving the boss about and lots of wipes happen because he reaches the edge of the room and just kills everyone because you've got to keep him away from the edge.
Sometimes volcanoes spawn in the middle of the room, all the healers and ranged will have to be aware of it and move out of it or they'll all die on eruption.
Gauntlet trash and 25m boss fight video http://www.youtube.com/watch?v=5XfPrbmp ... detailpage