It's a cool concept but a bit too slow. With a game like this where the player has to retry a lot you want them to get back to speed as quickly as possible. But in this game it takes a while to kill a monster. So if I died in level 6 it's gonna take me a while before I get back to level 6, giving me only 5 upgrades. Instead of making the player mana-hungry at the start, I'd suggest finding a way to make that problem appear as you progress. Perhaps the damage you deal is equal to mana-consumption?