Two pieces of advice:
1. Make the play area two-three times the size, and increase projectile speed by about 3/4 (1.75-2.5 times) as much to partially account for the difference. First boss, if you were above/below it, you were hit at about the same time as you could register the shot had been fired. So more space to see it/dodge it, but more speed to not reduce it's danger TOO much.
2. For the third boss give SOME indication of where the beam is going to be. Twice I was killed while moving because the beam formed centered on my position.