Duke Nukem 3D: 20th Anniversary World Tour

This one is relatively easy and will likely be the first achievement you unlock in the game. The easiest way to get this is to start a new game and fall down the chute. Kill the enemy on the crate in the distance to your right and hop onto it then hop over to the ledge to get the rocket launcher. You'll get this one working towards the Oomph Uugh Where Is It? achievement.

If you really need help, 3D Realms has a full walkthrough available on their website which you can open in your Steam browser in game. It is located at http://legacy.3drealms.com/duke3d/walkthrough/