I liked that there were a variety of enemies, but I found myself using the same strategy regardless: get the rocket launcher, then stand in the middle where that one ammo piece keeps spawning.
One suggestion I have would be to make weapon pickups more temporary. For example, you could replace the ammo drops with the most basic gun, and every time you pick up a weapon, you have two shots before you have to throw the weapon away (possibly another projectile :O ) and pick up a new one.
Another thing that might make the game more fun to play would be increasing the mobility of the character. If the ammo spawns on the otherside of one of the big enemies, you can't do anything and just die (assuming you ran out of ammo). If the player was more mobile, by increasing the jump height or maybe wall jumping, or they can stand on enemy's heads for a brief moment for example, you could overcome these obstacles.
I'd love to see where you take this :)