Duke Nukem 3D: 20th Anniversary World Tour

When you start the game, you have access to every non-secret level in the game. To unlock this achievement easily, just play the last level of the episode and defeat the boss.

Select Single Player > New Game > L.A. Meltdown > The Abyss then your difficulty. Then, while in this level, enter the cheat code. Turn ‘God Mode’ on, ‘No Clip’ off and select ‘Give All’ and ‘Show Map’ (then you can press twice if needed for a top-down view so you don’t get lost walking through the walls). You will need to enter the cheat code for each cheat you want to activate.

The video below shows you the quickest way to get to the boss, the Battlelord. Defeat it by any means and the achievement will unlock during the end cutscene.