For those that have played any of the past Uncharted games, you will be familiar with this difficulty. It is difficult, but unlike other games, the enemies are not super charged. You will die very easily, but enemies also die quite easily. Headshots are the best to quickly take enemies out. Make sure you take cover whenever possible and never leave yourself exposed to enemy fire. It only takes a few shots to die.
Once you get the hang of things, crushing difficulty in Uncharted The Lost Legacy, is not that bad. Just like how it wasn't too bad in past Uncharted games. Clear enemies fast and take plenty of cover. The faster you get better at headshots the easier you will find playing on this difficulty.
Unless you are super caught for time, it is probably best to play on crushing difficulty on your second playthrough. This way you can use cheats to give you unlimited RPG ammo. The game is a hell of a lot easier when you have an RPG with infinite ammo!