The difficulty seemed about the right level, but after about level 3 it would start telling me I'd beaten the level after only killing a subset of the enemies, rather than all of them.

Pretty easy to crack once you run around a bit. The enemies just kinda line up and you can pop 'em off one by one. Otherwise it's a pretty simple game, but it moved along fast enough that I didn't mind restarting when I died.

Those green ones though, holy hell were they fast...