Fun concept! I like it. I slightly found it a bit too easy, or maybe I misunderstood something, but possibly the difficulty ramp up should be a bit quicker?
Man, I'd love to see how far you got because I've found it quite the opposite. Best score I've been able to get is a C. I find that you have to really power through levelling quickly or the difficulty overwhelms you. If you ramp up the difficulty, you will have to give the player much more XP to compensate. Mid to late game is just impossible due to health regen being so sparse.