The hardest part about this ACHIEVEO is that you'll need a second person to DIRECTLY DAMAGE all [assistants]. Keep in mind this, the main person will train everything and do his / her best to stay alive healing themselves (if you have a merc healer it might die) while the other will kill the [assistants].
There are a total of 16 (I believe as I counted them down) assistants. Once you kill 7 of the 16 you will get the ACHIEVEO.
Feel free to lay waste to the rest!