The first think you need to do is disable auto-save! To get the sharpshooter medal can be abit of a pain since you will need to kill everything with your machine gun.

I would recommend using the F/A-18F with the ECMP. At the very beggining of every mission change your SP weapon to ECMP so you cant fire off any missiles by mistake.

You basically need to bring down all your opponents with the machine gun, which can then be checked during the briefing. If your machine gun kills is less than 100% you'll need to retry the mission.

Remember: 1 Missile or SP attack and you have to start over, so be careful!