Before we start talking about the boss, note that the room itself is a part of the fight because a quarter of it will get flooded with slime. I’m not sure if it’s random as I’ve never paid attention to the timing, but you can tell if it’s about to get flooded by looking at the pipes. When ooze starts flowing out from them, the floor below is going to get slimed. Stay away from it.
Now, about the boss himself. Rotface is mostly a tank and spank except with a lot of movement. At the start of the fight, DPS away. He’ll cast Slime Spray at some point. Aside from the cast bar, you’ll know he’s going to do it when he suddenly turns away from the tank. The simple rule is this: if he’s looking at you, things are going to get ugly if you stand still. Move to his side, get behind him, etc. Just don’t be in front of him when he sprays.
The only debuff you have to watch out for is Mutating Infection. Like most debuffs, it’ll cause damage, but that’s not what you should worry about. When the it expires, it will spawn a Little Ooze that’s stuck on you until you merge it with another Ooze. What you should do next depends on the following:
If there are currently no Oozes other than yours, you’ll have no choice but to wait it out until someone else gets the debuff and spawns another Little Ooze. Bring them together and they’ll merge into a Big Ooze.
If there’s a Little Ooze on another player, bring them together so they’ll merge into a Big Ooze.
The Big Ooze is tanked/kited by your raid’s off-tank. If you get a Little Ooze and there’s a Big Ooze present, bring yours to it so they can merge.
When the Big Ooze gets enough stacks (I think it’s five), it will animate as if it’s boiling and then explode. There’s an emote announcing this so just watch out for it. Your off-tank should be able to call this out as well. When the Big Ooze explodes, it will send a bubble of slime to each raid member. It arcs in the air so there’s travel time involved, giving you a few seconds to move out of the way. It will land in spots where players were at the point of explosion. Simply put, the Big Ooze goes boom, get out of the way and make sure you end up in a spot where nobody else was during the explosion.
Here’s a little trick for this fight. While you have to run during the explosion, you don’t necessarily have to run away from the boss. Picture the fight: the boss is being tanked in the middle of the room. The explosion occurs. Everybody turns 180 degrees and runs. Your main tank does not have this luxury. Instead, he starts walking backwards, kiting the boss. You can follow him or her as he does this as long as you make sure you move along a path that no one else was on during the explosion. This is easier to execute if you’re attacking the boss from the side and not directly behind him. Why? Because when your tank moves backward, you don’t end up moving forward and running along the path he or she was just on. You just move sideways.
I’m not sure if Feint will help minimize the damage from the explosion but give it a try and let me know.
TL,DR:
Stay away from the slime-flooded area. Don’t get hit by Slime Spray. Merge the Little Ooze with another Little Ooze or the Big Ooze, whichever case applies. Watch for the explosion and move away from the raining slimes.