Bonus Ending -
To get the Bonus ending you must successfully complete the game and save. Then load the saved game to play any level. You will also be able to able to play as some of the monsters or
Bosses.

After you complete the game once, save your game and then when you want to play again, reload it and you will be able to play any level and also play as some of the characters in the game.

Ending bonus:

Complete the game and save it, then load the saved game and you'll be able to play any level. Alternatively, you can play as some of the monsters or Bosses.