how about a sandbox mode, with a dropdown to spawn things and infinite upgrades. Then you can practice different upgrades on different enemies, to get better at the game without being totally destroyed before you can try. Of course, the unlockables, such as power kick and flame breath, are still unlocked until you complete the challenges. Perhaps even you just choose which enemies to fight, and the game makes you a match against them and you get an upgrade per round as normal. No "High scores" for if people do one sword robot repeatedly.