Difficulty seems too high for the first stretch of the game before any power ups. Becomes much easier once you get a couple of the power ups, the x2 and random skulls in particular
Depending on scope of this game, I'd suggest...:
(For PC) Instead of having to hold the mouse down, maybe add right click to snap select/de-select as desired.
Varied color. The minimalist color thing works well with red/black/white in general and the enemies were easy enough to distinguish based on size and shape alone, but I'd suggest color change/(tweening blues, greens, etc) to remove some of the monotony that comes with this style of game.
Overall I think its a pretty neat, tightly packed, juicy game with good polish.
Am curious to see what you can come up with using this level of quality on a bit larger scoped game in the future