Call of Duty: Modern Warfare 2/Overwatch

Walkthrough
The higher the Star level, the harder the enemies, the more time you have and the more distance you have to travel. This mission requires two people and it helps if both people have a microphone to communicate.

Ground Troop
Start by looking at the timer and going down the hill until it starts, then go back to the starting position and hide behind the wood. Wait until the AC-130 Gunner has killed all the troops then proceed. When you get halfway through the barn, another wave of enemies should spawn, aim at the doorway and kill them all. Now you can risk rushing to the right and getting to the next building or you can wait for the AC-130 Gunner to finish off the remaining enemies. It is fairly safe to run as long as you point your laser designator where you're going so the gunner can help you.

After you get into the basement, you will have to face the hardest part of the game, either to the left or to the right there will be around two enemies. As you are inside your gunner can't help. Throw a flashbang and clear out this building then continue when you are told the way is clear. Continue and go around the left of the building. Go forward past the bin to spawn the next wave of enemies then retreat and leave the gunner to finish them off.

The next field should be easy if you go around the outside, sprint towards the greenhouse and go prone in there until the gunner has cleared the way. Go out but be ready to retreat if any more enemies spawn. After they are all dead, you should be able to finish the mission with a minute or two to spare.