True, you don't want to make it too easy... what if you tied the movement of the paddles to the mouse so that the up and down keys could be used for your projected angle? Otherwise if it is too easy you could always try to influence the direction opposed to setting it. Just some thoughts.
The invisible enemies were the moving type, and I believe that they started spawning after the Red moving enemy tier. It was pretty late in the game, I think I maxed out most of my stats lol Kinda hard to put it down when you can keep leveling up :D