This achievement is simply completing every level within the campaign, on any difficulty you prefer. As long as you aren't on Ultra Nightmare the difficulty can be changed whenever you want, however any cheats you unlock within the game can be used without disabling Achievements. To that effect the first level, Hell On Earth, has an unlockable cheat for infinite lives. This essentially trivializes the difficulty of most if not every portion of the game and with the exception of Slayer Gates this can be used for every portion.