A lot of glitches
The game is a lot of fun once you get enough skulls but getting there is pretty tough without double jump or the extra health. I think you should lower the skull requirements a bit for some of the stuff and get rid of the wall jump skill, there's no point in upgrading it at all once you have more explosives.
Also some more instructions would be nice, It took me a while to realize I can still kill the slimes but only from the bottom which makes me invincible for some reason as I fly up.
Also, please get rid of the ads or something, I accidently click on them so much it really gets on my nerves, they also sometimes randomly come up WHILE I'm playing the game.
Hit boxes need a lot of work, I fly through enemies or get hurt from enemies when I'm nowhere near them, the one extra large blue alien I think hurts me when I'm on the other side of the screen.