Nice game ! I like the "defend your partner" principle 
It's very polished. Also, the game is easy to understand. The music is nice and creates an interesting atmosphere, however the sound design is too repetitive (may be add more sounds and apply a pitch modulation ?). The choice between repairing and upgrading is very interesting as it allows to modulate difficulty, good idea ! Nice to have random upgrades available, so we want to see what the next one will be ! To enhance the game, I noticed the fire shot doesn't fit exactly the mouse position, which can be frustrating sometimes (when it touches walls). Good job !