A LOT of room for improvement.
I'll go through, level by level, what I think could be better
First Level- It was a fun level, but you need to put a better restriction on the number of on-screen enemies. My computer isn't the greatest, but I had it on lowest quality, and it was lagging so much I thought the computer froze.
Second Level- I think it could have used a bit better control, like maybe instead of bursts you could hold the down button for more air.
Third Level- My favourite level, but only because I'm such a fan of Indestructotank. That and I had a little bit of the same lag problem as level one.
Fourth Level- Way too fast. It came to a point where I was just hitting jump frantically, hoping I didn't die, which luckily I didn't... by about 3 health.
Fifth Level- Not much strategy involved. Generally, I like for boss levels to have a strategy other than smashing the attack button. Even something as simple as an invincibility dome that you need to shut down or wait to shut down (doesn't fit the theme very well, but you get the idea.)