Anyway, Bungie stated ages ago that boosting Gruntpocalypse and Campaign won't reset your credits. I've got 1000 in the game as well thanks to this glitch for the last achievement and the Mods here and on Bungie said it's ok, so I won't be playing it anymore - the multiplayer maps and modes at the minute suck ass =) I may even buy Medal of Honor just to have something to play until Black Ops comes out I'm that bored of the Xbox at the moment, lol.
Yeah, it's not too bad, the later levels do have some annoying parts though. Bungie site is really slow to update, I still have default armour on there, apparently they are fixing it with the October update. My video is in my File Share as well, one on Gruntpocalypse.
You need to play Solo Score Attack and keep vetoing until Gruntocalypse comes up on the Corvette map. Stay in the opening tunnel as the Grunts can't come in. Get lots of headshots and sprees and you will get between 1000 and 2000 Credits per game.