I started my low-level run a few days ago. I only fight required battles as far as I know. I'm really good with saving up items and being coin-effective. What I really need to know is this: what are the lowest levels possible after getting each Star Spirit, then right before the last Bowser battle? Also, what if any battles that are required to advance in the story or get a key item can be run away from or avoided? (For instance, is it possible to not fight the Bullet Bill Cannons at the Koopa Brothers' Fortress?)
Thanks.

I think i managed to jump over the cannons once