Hint: Medals:

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

Hint: Easy mission completion:
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.