You can get this achievement on any of the difficulty setting so if you are new to the Call of Duty series then feel free to play through on "Recruit". This is a very short mission and should only take about 10 minutes. You start off by holding back the enemies across the river while the bridge is being constructed. You will find a group of them appear on the bridge and once you have finished with them you will be able to cross. After a short drive you will be forced out of your vehicle and have to proceed on foot. Follow SGT Foley till the end of the mission.