This can be obtained in Story Mode or Freeplay.
A short way into the level, Arrow will obtain his bow. There will be a few spots afterward where enemies appear and you can shoot them. Unfortunately, Slade is pretty good at killing enemies, so it's unlikely you'll kill them all.
No matter, though, just continue through the level until you come to the gate of a military encampment. Just inside the gate to the left is a tent, and then another one is a little further back on the left behind the first one. If you enter the gate, be aware of the enemy shooting you from atop the stack of crates on on your right.
These tents will infinitely spawn enemies until you zip them up, so go to town. But keep in mind that they all tend to rush you, so make sure you give enough distance that you don't melee them instead.
After you kill 20, the achievement pops.