Just a suggestion: After each phase of the fight, have the game refill your health. Other than that, it's a great game.
One of the upgrades restores your health and gives you one extra heart, maybe it didn't appear for you..? It's 3 out of 5 chance of appearing. Either way I think you have a point, starting a harder battle with less health can be frustrating. I might rework that in a later version. Thanks for the feedback!