The difficulty achievements DO stack so if you beat the game on 007, you will receive all 4 achievements. I suggest playing on 007 right away because it is not that hard.
Look here for a complete single player walkthrough written by Frankie B1.
- Make sure you use a lot of cover and when you see an ammo box, make sure you get ammo.
- Don’t rush into areas with shooting aimlessly, move slowly from cover to cover.
- Know where your enemies are by looking at your map. Press the Back button to view your map. Green arrows are enemies that don’t know you are there and are not on alert. Yellow arrows are enemies that don’t know you are there but are on alert. Red arrows are enemies that know you are there and will shoot you if they see you.
- Know the surroundings of enemies. If you see a gas tank or an electric box and you don’t have a good shot on the enemy, shoot the gas tank or box. The gas tank will kill the enemy and the electric box will make the enemy get out of cover and prevent him from shooting at you.