Fun game! Fun character designs, too, and just enough narrative to make things interesting.
I do think 30 minutes all in one go is a bit much, as Impy commented. If you want to continue working on this game, I think exploring a mode with multiple shorter waves could be interesting. Perhaps a selection of temporary powerups would be cool too, to add some texture to the gameplay.
That said, I think you have a very solid base here. I had fun with it!