Game is very badly laggy. This is even worse in the later levels. I recommend setting the quality to low even if you're using a high-end machine.
Now for a bit of strategy. Get to that alligator boss (first boss) but *don't kill him*! Instead, use shield if necessary, and shoot meteors only if it's safe to do so without hitting the gator too much. What you'll want to do here is literally grind up everything.
- Grind up rainbow ability so you have a huge rainbow meter.
- Grind up damage increase abilities so you're dealing more damage.
- Grind up all the lives so it not only helps you but also you get a medal for doing so.
- Grind up shield helper items
Once you have the rainbow meter to the far right as well as lives, now it's probably safe to go ahead and kill that gator boss.
Now enjoy breezing through the entire rest of the game, even killing bosses in one hit (if you grinded up enough damage increase items). It should be easy now.