You will face off against a one-time enemy in stage 28 called the Catalog of Doom. Stand far enough away from it and it won’t go after you at all. Stand at a distance and hit it with the whip and with other long-distance weapons and you will easily beat it without taking a single hit the whole time.
*SPOILER ALERT* (don’t read below if you want the final boss and what comes after to be a surprise)
On the final stage you will face off against two final bosses, one of them being Edd and the other being Fat Ian. To beat Edd, do the same thing as you did against most of the other evil donuts – dash in for quick hits, then dash back out before he attacks. He won’t do as much damage as the other bosses, but he tends to be far faster.
To beat Fat Ian, run in with your sword and attack, and watch how he wobbles around. You’ll be able to see a distinct difference between when he simply sways and when he is about to fall on you. The falling attacks will be a nearly instant kill if they hit you, but they are fairly easy to avoid, even without dashing. Simply run away when he is about to fall.
After you beat the game you’ll be able to unlock new difficulty levels. One of them will be the Sorta Hard difficulty, which is unlocked after you get 60 stars on the Kinda Easy mode. After you get a full 90 stars in the Sorta Hard mode, you’ll unlock Hard as F**K! The main difference is that you’ll get a different mix of donuts per level (harder mixes), and you will get slightly different dialogue at times.
Of course, oftentimes they aren’t even harder on certain stages, because for one thing, you will get to keep all of your weapon upgrades. Plus, donuts can still attack each other in the harder levels, meaning that they will simply do more damage to each other when you bait them into committing friendly fire.