Pit Boss Achievement
Show that pit who's boss
This can be gained in Chapter 9 – Chamber 17.
Wheatley tricks you as you enter this chamber and hit the launch pad. When you finally land on the platform surrounded by his death trap, aim and shoot a portal on the wall up ahead past him and then when the conversion gel hits you, put a portal under your feet.
Now that you're on the walkway, start to walk away from Wheatley. He'll then ask you to come back and jump in the pit below. Doing so, and killing yourself will net you the achievement.