Too Easy!
I don't understand how people are saying this is hard, I beat most bosses without barely a scratch (no more then 50% was lost, ever). However, your AI is a bit dim, if you stand on a box and they are on the ground, they will just keep running into the box while you pound them with your gun.
The game was fun, I particularly like the fire ball gun thing, though the sand box mode was slow, very slow... Overall, good job!
--Strategies--
Green guy who spits - Crouch and fire.
Melee Green guy - Get on a box and fire or run shoot and jump over him.
Tower - get to an equal level (if possible), crouch and shoot).
Red guy - same as Melee Green guy.
Dog - same as Red guy.
All the bosses have an easy way to dodge their attacks be it standing on a box out of their range, timing your jumps, or positioning yourself properly.