A simple game the fits the theme well. Not doubt as you've heard from countless people by now, the cooldown between shooting different weapons really hurts the pacing of the game. The decision to include a cooldown makes sense, to force the player to make plans and to position themselves well with the arena, but the wait is WAY too long.
Besides that, the grenade weapon seemed a little difficult to use, while it's one of few weapons that works against all enemies it launches way too fast and bounces way too much to be of much use a lot of the time.
Other than that, it's all positive, I love the concept, executed well to make a simple yet entertaining little 2D shooter. Good job! ^-^