Crushing appears to be disabled at the start of the game, but it actually isn't. If you go to crushing difficulty it is grayed out, but if you press X you can select it anyway. Normally in past games you were required to beat the game first to unlock crushing difficulty.
Crushing difficulty is the hardest difficulty in the game. Enemies are tougher and you die a hell of a lot easier. You will no longer be able to brush off grenade explosions and meele encounters are not going to be a walk in the park. If you have played the game on crushing mode for any of the past uncharted games, you shouldn't get too frustrated with this. It's a challenge, but you will know what to expect having completed it for other games. If this is the first time playing crushing difficulty, here are some strategies and tips for beating the game on crushing mode.
Get Headshots - Headshots are the quickest way to kill enemies. The quicker you kill the enemies in an area, the less chance you have of getting killed by them. You will also burn through less ammo as the enemies are notorious for being bullet sponges.
Use Stealth - If you can sneak up on an enemy and take them out using stealth then you should. It is better to clear off as many enemies as possible using stealth so that if you do finally get caught, there will be less enemies to kill.
Take Cover - Running into an area with guns blazing does not work. Take it slow and use all the cover you can. When you want to move somewhere, make sure there is a plan b to run to if you get flanked or something.
Keep your distance - You dont need to run into the middle of an area to start taking enemies down. Stay as far away as possible and try to pick off as many enemies as you can before moving forward. You are less likely to be flanked of the enmies are further away.
Choke Points - If you are behind cover and get flanked, you will most likely end up dead. Rather than go inside a door and take cover, stay outside. The doorway will act as a choke point. All enemies will need to come out of this in order to get to you. This allows you to easily pick them off.
Use ledges for cover - Sometimes you may be in a point where there isn't much cover. What most people forget is that you can drop off a ledge and hang there. This is the best kind of cover as your entire body is hidden. Nate being the beefcake that he is can easily pop up and down from hanging off a ledge to pick off enemies.