This achievement is really quite simple. It is obtained a fair bit into the level "The Tower", the 4th level of the game.
Before this, you will be fighting on top of an elevator. After fighting off the enemies you will then have a change of scenery and come into a cave area.
Once in the cave, you will walk forward for a few seconds until your path leads to the left. What you should instead do for this achievement is to keep on walking forward, and drop down. This will have you take damage so be aware. That's it.