Conscriptinator

After the first three bosses died, don't jump down to the last boss, but turn right at the spot, where you normally would jump to him. On this wall 3 Mantid Conscripts are fighting NPCs.

To get this achivement, simply kill those 3 Mantids and stay near them, when they die. They explode and give all targets within the explosion a debuff [spell=118795] with a 90 second duration. In addition these adds have a stacking buff on themselves, increasing their damage, so you do not want to face more than one at a time.
As soon as you got the third stack, you will recieve this achievement.

Level 110 Solo Version:

- Before jumping down to last boss, look up at the stairs to the right. You'll see Krik'thik Conscript fighting NPCs.
- Run up to the first Krik'thik Conscript and get it to follow you.
- Run to the left into the tower, get the 2nd Krik'thik Conscript to follow you too.
- Run to the left out of the tower, wait for the Krik'thik Conscript that are following you to arrive.
- Do any multi-target attack to kill all 3 of them at once.

All done. :)

Just as a heads up if you are a dk, don't use AMS as you won't get credit for the stack.