Game mechanics are fun. I'd suggest making it impossible for enemies to spawn in positions where they immediately attack the player without the player being allowed any actions in the infinite arena, but then I suppose you could just keep going forever. You probably still can with the shield, though, assuming it recharges between waves. I took the jump damage ability and the jump range increase and that made the rest of the game pretty easy after the second upgrade. Still fun, though, and I think that's probably pretty lucky of me to get.