Defeat Boss 125 in Evil Difficulty to unlock Greasy Nerd’s Adventure zone
Enter the Greasy Nerd’s Adventure zone and defeat the Greasy Nerd’s Mom
After defeating the Mom, you’ll need to rebirth and enter in the Greasy Nerd’s five letter code, with one letter appearing under certain bosses’ story text. You have to press each letter on the boss that it appears under while you are facing that boss in Fight Boss
Turn off Auto-Nuke if you’ve previously obtained it. Make sure not to Nuke bosses, if you defeat a boss that has a secret letter, you’ll need to revert to a past save or rebirth again to unlock Greasy Nerd