Contains two major dps checks, and two jokes.
You will need two tenacity pets, with shell shield on auto-cast and cower on manual cast.
The goal of this fight is to handle pet healing/switching while dpsing enough with a screwed up rotation (12K needed).
Pull Patchwerk by misdirecting him to your pet, then feign death to avoid pulling all the room with him. Keep mend pet up and dps as much as you can.
You can use 4 rapid fires during the fight. I usually use them when my pet uses shell shield, except the 4rth.
Since Patchwerk hits really hard, after a while your pet will use shell shield. Manually use cower when it finishes. Once your pet is near death, switch pets. You can use snake trap (pet on passive), but it's a dps loss; or simply run away (pet on assist).
Once patchwerk is at 7-6%, switch pets, and pop rapid fire/last stand/trinkets to kill him before he kills your pet.
Just. Kill. It.
Bug: to make the fight easier, feigning death will remove most gas clouds.
Second hardest boss in Naxxramas.
Legit Method: Tank Gluth in the back of the room. Unless your pet dies before resetting his health.
Dps Gluth whil maintaining your pet full life and dispelling his enrage. Kite the zombies using a frost trap (run around in circles or do a horseshoe maneuver with disengage at each end). Once your pet is at -100% healing, pop shell shield, then cower. Once he's about to die use last stand.
Make sure the zombies are far from Gluth when he decimates. Kill them all, then switch pets (use of snake trap is recommended to avoid Gluth eating a zombie). Also try to destack yourself.
At the second decimate, either:
-Gluth is low on health: finish it!
-Gluth has too much health: kill zombies, pet switch.
Bug: Probably the greatest exploit in all Naxxramas, transforms Gluth into a high - hp trash.
Tank him near his spawn point (but not too near). Dps him. Once you have 3-4 zombies, dismiss pet and fein death. Wait for Gluth's health to reset. Then, as soon as he spots a zombie, jump. Tadah, Gluth without zombie spawns (nor decimates). You can also destack your pet by taunting Gluth at the right moment.
Attack one of the two wights, on their platform. Dps it while the other runs at you, and feign death before the magnetic pull. Then put your pet on the second wight. That should avoid you taking damage from the tesla coil (unless it aggro's on your pet, too).
Kill them both at the same time, dismiss pet and jumps. Resummon pet and misdirect Thaddius at him. Then kill him.