Bosses cheese:
Whenever a boss is shooting triangles missiles or returning to center (pink and orange phasees) spam the left click towards the center of the boss (orange phase).
However when the bosses are shooting you should aim for the corners (much easier with the square where you just follow him hitting the same corner over and over again). In the case of the circle try to make a star shaped drawing with your character while hitting the boss that way the missiles hit him and you hit him a lot as well.
You can end the battle in 30 seconds almost always and with a bit of luck under 10 seconds.