It's the 500 kill challenge. If you aren't familiar here are the rules:

FFA Nuketown 30 Health Max Bots (I think it's 18) Max Class Points (I think it's 17) Unlimited Score 10 Minutes (so we can use in game time)

Here's Ali-A's video:

Since it's Hardcore health, the guns I would recommend are the Weevil, HVK, and the Dingo

This would be something suited for the category extensions board, not here