I made it to wave 7! This is a fun game.
Here are some suggestions:
It feels a bit repetitive because there is only one map and the same ennemies spawn in every waves. Waves with different compositions would add diversity without requiring a lot of work.
A special player unit with a different weapon would be cool too.
Playing a sound effect when the big ennemies are hit or die would add to the juice of the game.
The characters (players or ennemies) tend to stuck themselves in inward corners (pathfinding/physic bug?).