1. After all your challenges are fully reset then start a Score Attack game in matchmaking. When the game starts pick the shotgun class and get 19 kills with it so you don't unlock the challenge "The T.G. Opposition" but your one kill away.

2. Get killed somehow and get another gun of your choice. Get 49 kills with anything but the shotgun so you don't complete the challenge "Where there's Smoke.." either.

3. Get killed until the game ends then start up another Score Attack.

4. Get a kill with a shotgun which will give you 802 credits (500 for 'Where There's Smoke...' challenge, 300 for 'The T.G. Opposition' challenge and 2 for a kill).

5. Unplug your ethernet cord and instantly plug it back in (If you have a lag switch you may switch it on then off right away). Then your challenges will go back to before you started the game and you will still have the credits. Hit the guide button and sign back into xbox live.

6. Wait until it says how many friends are online then repeat from step 5.

Haven't tired it yet might get you banned tho lol.