I'll need to go back and kill that first boss either way. Like I said, it was close. I have a couple damage modifiers that increase my dps output as my health goes lower. I need to take advantage of that.
In the town on the island, there's a crystal vendor who sells crystals you can affix to your weapon and armor, if you haven't done so already. Crystals can be removed at will (only while still equipped), so you don't have to re-buy them for every upgrade. Anyway, there are some weapon crystals that give you MP regeneration, and armor crystals that give you HP regeneration. I think the level 11 crystals are something like 10 HP every 5 seconds apiece, and you can usually get armor with 3 slots (maybe only 2 on the island).
Also, I'm sure you realized this well before I did, but that dash ability you get on Warrior costs HP as well. I was using it to get around faster before I had a mount, and was wondering why my health was always so low.