Use the following trick to complete Missions without doing the objectives. Play the mission like firefight mode. Once you kill everybody, the mission is automatically complete with all objectives done.
Not really a Cheat, but cool to have:
Purple Heart: Get wounded or killed in action.
Bronze Star: Get 15 kills
Silver Star: Get 20 kills
Distinguished Service Cross: Get 25 kills
Congressional Medal Of Honor: Get 30 kills
Complete the first 12 missions in Campaign mode under any difficulty to unlock 12 new soldiers as plalyable characters.