Side quest before finish the game?

Hi, i just beat Seymour(4th time) and i'm about to fight Jetch, which i suppose is the final boss(hopefully they don't do a Necron)

I can't defeat him, i can barelly get to the second phase. So, becouse i didn't do any of them, i would like to ask what is your suggestion for side quest to do, both to get stronger and becouse i didn't do any of them during my game and it fell like a waste.

Plz, don't give me any suggestion on how to beat jetch or any other boss itself, I'd rather trial and error or go back to farm than look at a guide to beat a boss.

Thanks you all.

Sorry for bad english