This assumes you made it to Level 10 which is the last level for the American world and it's a boss level.
You need to make your way to the top of the level, crossing over moving platforms and avoiding enemies AND a gun shooting globs of mustard and ketchup from the right side of the screen. The globs kill, so jump over them, or make the gun shoot from a different height by getting it to lock on you, then you move up or down. The globs also kill enemies, so use that to your advantage as well. Once at the top of the level, hop across trap doors and a breakaway floor to the left side where a pickle enemy waits and the top hamburger bun. Drop the bun and work your way back down, dropping the bun each level while avoiding gun globs and respawned enemies. When a burger is finished, it conveys into a machine at the bottom and damages it. It takes 3 finished burgers to destroy the machine and complete the Boss Level (and get the cheevo). So you have to repeat the same steps. It took me a few tries, but it's doable without going down to Easy mode. Enjoy.