Come on guys, I am surprised to see so many game overs on level 6, mainly in the "Circle around the tower section".
You just move up then down then up then down to get rid of the first batch of enemies.
And the "boss" is incredibly easy. Just go all way to the right, and his shots won't hit you. When he stops shooting he will rush at you to try to punch you... you just need to tap a little to the left, and when he moves back to the top of the screen, move all the way to right again. Repeat until he explodes
Easier than Sonic 2 last boss
5th boss isn't hard too, but maybe with increased rank it may give some trouble. I usually would reach it without any lives, heh.
Boss 3 would give me trouble sometimes (it needs much more adlib-dodging than level 5 boss), and level 4 mid-boss is the one to be feared IMO. I played it enough to get most of its patterns right, but when the "little helpers" open up to fire the lightning balls at you... THAT PATTERN would kill me ALL THE TIME. I think I only dodged it once without getting hit (Sometimes I was lucky enough to reach that pattern still wearing a shield). I just couldn't figure out how to get past it without problems.
The only way I could actually beat that mid-boss was to destroy it quickly enough before he could fire that pattern for the 2nd time.