Level skipping lets you use a completed character to help a new one unlock the final stage without playing the whole game. By running only key levels with checkpoints using a high-level character, you can carry the new one through quickly. Once enough checkpoint stages are cleared, the final boss unlocks and you get the skull for the new character.

1. have a character with the game completed
2. setup a co-op or get a friend with the character you wanna level
3. go through and complete EACH checkpoint level.
4. after you have completed the final level you will get your skull and or character you unlock from it

Now what are actually the checkpoint levels:

- Castle Keep (Entering Barbarian War, beating the first wave of enemies, walking into the left exit and then exiting to the right counts as beating Castle Keep)

- Barbarian War

- Barbarian Boss

- Thieves' Forest

- Catfish

- Pipistrello's Cave

- Wedding Crash

- Parade (Entering Cyclops' Cave, walking into the left exit, then walking right into the cave counts as beating Parade)

- Cyclops' Fortress

- Lava World

- Industrial Castle

- Pirate Ship

- Alien Ship

- Desert Chase

- Sand Castle Roof

- Corn Boss

- Medusa's Lair

- Ice Castle

- Painter's Room (Wizard Castle Interior)

- Undead Cyclops' Room (Wizard Castle Interior)

- Final Battle