We all know the 2nd boss "Big spider that drains life of all the adds"
Well anywayz the achievement is to kill him before he gets to the top and closes the doors, so anywayz as 99% of people cannot sustain the dps to do this, there is another way around it.
First off you need a mage, this mage will invis down to the bottom where the boss is and agro the boss, and hide up in the far corner this will put everyone in combat so the group needs to stand near the entrance and zone out and back in to get out of combat.
Now that the group has zoned back in and the mage is the only one on the bosses agro table, the group then heads to the middle of the bridge and then the mage goes invis dropping all agro on the boss, hence making him despawn and then you have 15 seconds for the group to jump off the bridge, and run down to the spot where the boss spawns near where the mage is.
After the boss respawns you have 30 seconds to engage and kill him before any of the adds arrive at the bottom so it's a pretty easy achievement to get doing it that way.
Edit, Clarification for the mage.
Basically when you get to the bottom run around the pillar on your left and go to the far left corner, make sure you stay out of range of web