To earn the Pathological Cheater badge, use cheats 50 times. This achievement can be slightly finicky. Loading a space stage save game, cheating, and then quitting without saving 50 times will earn the achievement. However, starting a new space stage game, cheating, and quitting without saving doesn't work.
If the game has been affected by the cheating glitch, the achievement can only be obtained if none of the save games have the Joker badge. If a save has this badge, it must be deleted to earn the Pathological Cheater achievement.
See the Cheats page for complete list of Joker-Badgeable cheats.
Using the "addDNA" cheat 50 times also works, and it doesn't affect a Space Stage game.