As trophy says, you can finish the game on any difficulty. This is not a very difficult game at all but I recommend playing one time through on normal or easy. The game is about 5 hours long and is fairly linear and straightforward. The hardest part is the final boss. Some tips for that:
The boss is blind and relies on noise to find you so use the silenced pistol to shoot
The boss can only be injured in the mouth.
Killing the boss will take a few tries just stick with it.
Running around gets you killed, stay (relatively) still and only fight soldiers that are immediate threats the boss will take care of the rest.
Eventually, soldiers will stop spawning just leaving you and the boss, the best weapons I found are grenades, silenced pistol, flare gun and the high powered rifle. Grenades need to be thrown spot on as well as flare gun. With the rifle he can hear you so you must shoot then move to avoid being hit.
Good Luck