G.I. Joe: Operation Blackout

Play the game on Easy (Action Figures) getting all the collectibles, story based achievements, and doing all the side missions. This way you unlock all the modifiers. Replay Mission 0-17 on "Real American Hero" with the positive modifiers to make this fairly easy. Difficulty DOES stack so only 2 play-throughs required for the easiest and quickest route to the 1000gs.