You might also want to consider making the heal spell cost MP to use or find a way to reduce the amount of experience it gives since you can readily abuse the skill on the first map in order to get a unit up to level 20 before even getting to the second map.
I suppose this does mean if you had put a unit upgrade item in the shop as joke that it would actually be useable and allow the player to get a 20/20 unit before the game was over.