Haha it's technically possible to beat the boss at 0 without the flame sword, but you'll need to kill everything, and get many many upgrades and lots of health.

ALSO it's important to know that this game is not meant to be played like a single player story - it's a roguelike. It's possible to be stuck and need to restart. You know how sometimes in games you can lose?

Shameless plug: the speedrun record is 37 minutes to beat the whole game starting with 0 stones, so it shouldn't be a big deal to restart. Casually it shouldn't be more than 2 hours for a full playthrough.