To get Premium Reward 6, Unlock the other planes of your five wingmen. After this get the "Best Guy" or "Predator" rank in multi-player mode. When this is done, go to Challenge mode and select "Arena". Shoot down enough planes so you get the rank "Top Gun". After you get that rank, land and save. Go to Big Boss mode and the 305th Squadron. The F-15 should be available.