Just choose the Elite difficulty and either start or join co op games, this makes it much easier because you can respawn, and you also have some help cleaning up the enemies. You can also Play with friends who are playing on lower difficulty just like in gears of war, you can kill as many people as you can but kinda stay out of the thick action and let your friends do most of the work. Keep a close eye on your missile counter, and kill the easier ground targets and slower air targets with cannons you will want to save your missiles for dog fights. You can keep track of what missions you have played through on elite in your pilot records the mission hexagon will be gold.