For the spider boss you literally just sit in a corner until his final phase to avoid getting hit, that's it. It's pretty silly. But yeah the final few bosses outside out SA-X and the one that slows time they are really underwhelming/frustrating, agreed.
The 'overly-hard hitting' part I kind of have to disagree with because everything hits hard in this game, probably to highlight the predator/prey dynamic going on in the game. Because of this a lot of the bosses follow a simple pattern that you just need to figure out (e.g. jump into right corner to avoid dash move, then shoot rockets at weak spot. Rinse and repeat.) Not saying you should find this fun, I think the bosses are the weakest part of the game, but outside of a few they are generally quite easy to learn and overcome if you use the tools available to you.