as Darth Bieber said, the birdlike creatures flock attacking you are the most anoying and best for this achievement.
but for me took almost, 2 minutes. I was with Levv on the second level, the one where you destroy the poison clusters things.
on the part where there are some spiral stairs, using Levv ability I charge into killing the first summoner and then got ambushed by all the flying creatures, so then I remember, maybe I can try getting the 100 blocks. so they pushed me against one corner, without any other enemy attacking me except for them, and pop... pop, both achievements in on hurry.
hope it works for all of you too
edit: some syntax, Jesss