Just for the sake of making sure I have everything right for this encounter, and please post feedback if I'm wrong.
Group Set-up:
Main Tank
Off Tank/Back-up Tank
2-3 Healers
5-6 DPS
Phase 1:
Kill the slimes as fast as you can. If they hit a player, they take massive damage. Green Slimes can be slightly healed through, Red Slimes = wipe. If you turn into an abomination, suck up the green pools, and damage adds.
DPS, DPS, DPS until 80%. Green Slimes stun the player, Red ones do not.
Phase 2 (80%):
He stuns the raid at the start and gains the Choking Gas Bomb ability. Pretty much the same as phase one, but he puts two flasks down that do damage if your standing at it. DPS, DPS, DPS until 35%.
Phase 3 (35%):
He stuns the raid, drinks a potion (which does not heal him) and turns into something that does more damage. He doesn't spawn any more slimes, but he throws slimes at the raid which slows movement and attack and casting speeds. He stacks a debuff on the tank which injures the raid rapidly. If the tank dies, he is healed for the amount of stacks that tank had. To reduce raid damage, the other tank should grab him off that tank, when the time is right. Stay out of slime pools still, watch Choking Gas Bombs and kill him. Heroism and any cooldowns should be activated at the start of this phase.