Great game, finally beat it. Persistent upgrades would be nice (saw this in the Steam version - runes).
For anyone struggling, the build I used:
- Flame heart (0.5% chance to gain health when burn enemies)
- Fireball every four shots
- Run around to give off flame bursts
Works because lots of stuff catches fire and you regenerate health now and then.
Thanks to this combo, I survived until the end. Used shotgun + 4 shot upgrade + rapid fire upgrades too.