Left click to shoot, try to survive. Your score is constantly lowering, so kill as fast as possible if you want the most points at the end.
I think this is a really AWESOME game! I really like the concept of having to kill slime enemies to maintain your lowering score and the map is pretty well designed. Though I wish there were sound effects and a reset key in case you get softlocked. Otherwise, I really like the game a lot! Nice job. :D