Complete Story Mode on hard difficulty.

To unlock the hard difficulty, you must first complete the game on medium difficulty. There are a total of 31 main story missions in the game, so you must complete them all on the hardest difficulty. This will NOT be easy. Hard mode is very tough, I recommend that you remain hidden through as many missions as possible. If spotted, flee and disguise, avoid conflicts with stronger enemies like Super Soldiers and Leader Hunters whenever possible. If you follow the road map, you should be able to die and resume at a checkpoint when you do your hard playthrough.