This one is called "The Kid", and is awarded for unlocking The Kid from I Wanna Be The Guy. The warp zone is on 5-7 "The Fallen": It's on the right end of the stage, outside the second building, down near the bottom. You have to grab the first key, hoof it through the second building and out the window, then down to the warp before it closes.
The warp zone itself contains, by the developers' own admission, three of the hardest stages in the game. Clear all three of them, and you have not only a cool 10g, but also one of the most useful characters in the game. ;)
Here's a video of the warp zone (not mine), but be warned, the guy makes it look much easier than it really is...be prepared for hundreds of deaths on these levels.