Here's a simple character that I did out of boredom while I was developing IDW. They're quite easy to beat even on the hardest difficulty. Though, the same can't be said for the AI (some of them, at least).
Dinergates, a common enemy unit from Girls Frontline, swarm the battlefield with relentless numbers!
They'll chew through anything in their path... for a while and then move along. (just to be fair)
While they only move in one direction, they'll keep coming until you deplete their life bar and that can be done by taking them out one by one.