Ooooooooh, really good stuff here. I Love the idea that the darkness is literally eating away at the light. The way the background gradually gets darker is an amazing touch. And between the three weapons, you really get a good range of strategies.
But on that note, I think the first wave is a liiiiiiitttlleee too hard for the first thing a player sees. It's totally doable yes, but I feel like so many darkness balls come at once that it's easy to resort to button mashing instead of strategically thinking which attack to use. I think if you decrease the speed and amount of darkness in wave 1 and maybe 2, you'll give the player a sort of built in, no explanation needed tutorial to really get the weapons under their belts, so they'll be able to go into the full on darkness having their battle plan ready.
Well done with this! I enjoyed it!
-Sasha