To unlock this achivement you need to beat twenty waves of enemies in the arena mode. Play as the warlock as he gets respawns when his demon is leveled up, this essentially means you can have as much lives as you need. Before entering the portal to the arena mode play the first stage normally (play up until the castle door) this way you can get extra items and potions that will transfer with you to the arena mode increasing your chances. This method also works for endless dungeon mode. Save your healing items as they're rare in this mode.