Amazing! I absolutely adore this game. It's really fun to try and beat the boss with no upgrades.
Edit: Just beat it with one upgrade due to clearing out enemies before the boss.
With some careful planning, you can get past the enemies before the boss so you beat them without leveling up or picking up upgrades... I just did! But I collected 30 XP, and used dodge, heals, and bullets. Pretty sure you can do the whole thing on 10 XP if you get lucky, it's definitely possible to beat no-hit (+no heals), no bullets, and no dodge. Now I just have to try again...