Beat the game on the first 2 difficulties to unlock duckcore mode, where if you die you must restart the game.

Play through duckcore collecting all 10 pizzas along the way, and making sure to spare both minion and guard by talking to them instead of fighting. If you don't know where the pizzas are you can check out a guide or video. Make sure you don't click "QUIT" when it asks. Click "RETRY"

Now you have to go back, on any difficulty, and during the first level of fissure laboratories the door to the right down the stairs should be unlocked. Go through and shoot the big button, then run back up the stairs into the room you spawned in, and shoot the button above the doorway inside the room. This will unlock your final ending.