Boss Brawler

if you swap bosses during their death animations it counts as if you killed the one you spawned so you can kill any boss just by killing first one and spawning a harder one

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.

well, beat the game, bough the best gun and (obviously) defeated all bosses.
sure could use some more upgrades but they are likely in planning already anyways :-)