At first, you need to learn how to do the "flip" technique that allows you to get back in front of the boss. In the first boss, I explains how it is done (and yes I call it pirouette and it is wrong).

Also, I have not done a perfect good montage video but it's a good approximation of what and how you could do "simple" tricks on the bosses. You can always comment to improve the quality of this guide.

First Boss :

Second Boss :

Third Boss :

Fourth Boss :

Fifth Boss :

Sixth Boss :

Stage 8 - Fifth Boss :

For the last boss, I have not routed the game enough to find a perfect way to do it, you can always look at Sprintgod's video to find a way but it's really hard.

Stage 8 - Final Boss :

P.S. I've said a lot that Izumi have inspire the guidelines for the bosses but it's mainly due of his great activity on this game and he was probably inspired by other(s) speedrunner(s). (Same for Tsky)