Cool game and idea! Loved that you could create your own 'level'! But, I did find a bug :P
Got to the boss, accidentally got under him, reversed gravity, bounced off him and fell out of bounds :p
One weird thing is that you need to pay coins in order to remove HP potions, as you pay for a harder game. You could instead give the player coins, as if they're selling a good thing.
Other than that, really liked it.