I think the ONLY level you can obtain this on is the Swamp Arena, Which I think is just the Hard one, Can't remember but I think it's C just look for HARD not very hard.
I made life easier by having my friend help Co-OP just in case I got knocked down by a suicider or anything, Found the easiest way to kill the ram was using an Assault Rifle.
The Thug doesn't count towards this achievement for a strange reason so don't worry too much about killing them for this, If playing in co-op the one going for this needs to be the one to kill each type as it doesn't count for the other player if your partner kills a butcher in one round it counts for his tracker not yours, I used the co-op friend to kill all walkers etc carefully and if he seen any of these all at once he would run in circles until I was able to take them down.
If you've played the story and got to level 30+ this is a breeze since the enemies start at low levels.
Took me I think 15 rounds to get this :)