Algodoo

This is more-or-less how you would construct a side-scrolling game with killable enemies

The alien has twenty hitpoints. You can kill it with a machine gun that does one hitpoint per shot (already aimed, shoot with F), fire a big gun at it that does all twenty hitpoints in one shot (W), or drop two-HP blocks on it (numbers 0-9) until it turns into a puddle of blood.

Note: Multiple collisions at once over its hitpoint limit will repeat the liquify action, so getting it down to one hitpoint then shooting the big gun could crash it.