Hydrophobia

WOW. I just got this while locked out of the room because I reading how to get this achievement.
I just scumbag steve'd my way into Glory. :'(

This is a personal achievement, which means you're the only one who can screw it up for yourself, and no one else can screw it up for you.

This can be a tough one, and sometimes a little buggy at the start. They Hydrolance ability is the thing that appears to target 1/3rd of the room. The explosions at the end are pretty large, however. In fact, they're much larger than they might appear (10 yard explosion radius). Sometimes you can get unlucky with the graphic appearing at one place, but he turns a split second before the explosion, and thus explodes some other third of the room.

To avoid this, you should simply tank all adds and the boss in the back of the room. The puddle left by killing adds is not part of this achievement, so just go from side to side inside the door frame.

Once you get to phase 2, it's just jumping around in a circle. If you can keep it up for 90s-150s, you've got the achievement!

As of September 29th, just "AFK" before the first boss, tell them to go without you, stand outside the room, achievement, done.

Edit: As migit stated, make sure you're behind the wall.