Well when the slimes attack the player they bounce off right? That's the knock back i meant. By aggressive I meant fast, as they get flung back quickly then attack back at the player. If that was slowed down some, you could bounce the slimes into the spikes on the walls and kill them with out bullets.
For barrier hitting, when the slimes attack they hit the side of the barrier that makes them bounce. The defending sides. If they hit you "x" meaning some number. Perhaps 5? they get destroyed/killed. So like they injure themselves when they hit the defense side.