You will need to complete the game three separate times in order to obtain all of the difficulty related achievements. This shouldn't take too long, given that the game is fairly short. I highly recommend starting on Elites difficulty and working your way down. Elites will take you the longest, although the game isn't hard at all by the standards of the rest of the FPS game population. Simply stay crouched at all times, take out as many enemies via stealth (either kill moves or silenced pistol) as you can, take advantage of cover and take your time. Enemies can come out of no where at times. The AI is also incredibly stupid. The enemies will rarely move, even if you lob a grenade at them.