ALL LEVELS YOU NEED TO BEAT

If you didn't know, checkpoint levels are the only ones you actually need to beat in order to get a completion on a character.

Here is the list:
Castle Keep
Thieves' Forest
Catfish
Pipistrello's Cave
Parade
Cyclops' Fortress
Lava World
Industrial Castle
Pirate Ship
Desert Chase
Sand Castle Roof
Corn Boss
Medusa's Lair
Ice Castle
Final Battle

If you get a friend to play with you with a character they beat the game on you are able to just skip to these levels and beat the game way faster and easier. Hope this helps!