Interesting idea! I think that instead of a discreet XP value that you have to reach each turn, it might be better to have a total XP that slowly builds - that way each level becomes a balancing act of how much you're willing to risk killing the hero to get them just a lil bit more buff for the showdown. As it stands, I could just chuck a couple axes down at the start of the level and be done - I didn't really see why I needed to use the other units.