The boss on the Big Ass Donut stage (stage 5) is notoriously difficult to beat, but there is, in fact, a strategy to beat him. The strategy centers around the dash move, which the tutorial doesn’t even teach, so you will have to figure out on your own. The way to do a dash move is to swipe forward toward the place where you want your character to go. Swipe in any direction that you want and your character will go there. Make sure that your dash attack is perfected before the battle.
Use the dash attack when you get to the giant white donut, known as the White Jugger ‘Nut. The dash is the only way to avoid his jumping smash attack, so when he jumps in the air, start dashing around, and make sure you are in a dash when he reappears on screen in order to avoid his attack. To do damage to him, slash at him with the celery sword, then dash away when he winds up to attack.
The going gets tough when loads of donut enemies appear on the screen at the same time, but there’s an easy way to deal with that too. Due to either a glitch or a game design choice, donuts can actually do damage to each other, so when a ton of them appear at a time, run around and get them to follow you until they group up tightly, then turn around and run right through them so that they all attack you at once. They’ll all kill each other, leaving only one or two donuts left over for you to attack.
If it’s just a bunch of donut holes, another option is to dash back and forth through them. Donut holes are the only one of the enemies that can be damaged when you do a dashing move (the others don’t take damage from a dash). Either that or switch to a burrito bomb and launch a burrito at one of the donut holes, so that the blast from the explosion kills all of them.
When you get to stage 10, Churro Hero, this will be important to do as the boss, Jeremy the giant chocolate donut, unleashes a massive amount of donut holes, which can do you in quickly if you are not careful. Run in and attack him with the celery sword, then dash away when he’s about to attack. Repeat until he dies. Otherwise, another option is to have all of the donut holes try and attack him, just like above, but that’s a bit hit or miss.
Once you get a new weapon (which is after beating a boss, and a boss is once every 5 levels), go back to old stages and use it to clear out obstacles that you could not otherwise clear before. Sometimes this is the only way to find new stars.