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 > Lunar Apocalypse > Overlord then your difficulty. Then, while in this level, enter the cheat code. Turn ‘God Mode’ on, ‘No Clip’ off and select ‘Give All.’ You will need to enter the cheat code for each cheat you want to activate.

When the level starts, make you way through the tunnel and emerge from the water when it opens up. Directly on the opposite side of the tunnel you were in is a vent near the ceiling. This vent is only accessible if you have a jetpack or if you are using the ‘No Clip’ cheat. Shoot it out and enter the short tunnel. At the end you will see a large rotating gear (there will be an Octabrain floating around so you can kill it or not; it’s up to you).

Walk forward and drop down the shaft into a circular green room lined with eggs with a giant door at one end. Destroy the eggs so they don’t give you any issues (the Devastator works great here). When they are all dead, open the door to reveal the Overlord. Defeat it by any means and the achievement will unlock during the end cutscene.