Crysis

I found this to be an easier solution.

Load up the first level on easy mode, equip the silencer on both weapons and the tactical attachment on the scar. where you have to kill the first guy switch your bullets to the dart (tactical attachment) first, then go into camo and go to the guy, come out of camo shoot him then go back into camo. Then use the silenced pistol on his unconcious body, from where he was standing shoot the other two guys with the dart using the same method then go up to them and shoot their bodies with the silenced pistol. Repeat this method for the next two guards you encounter just after this small cutscene showing that dead nanosuit guy, aztek i think hes called. After shooting the 5th guys body with the silenced pistol the achievement should be yours, doing this my threat indicator did not move at all and i found this to be easier than the other ways mentioned.

Very easy achievement. On the level "Recovery", reach checkpoint with this objective "Infiltrate the school and locate the hostage". Find a precision rifle and look across. You should see a turret on beach. You can hardly see the guy, kill him with your rifle. WAIT!!! Another enemy should come to that exact same spot where you killed the last guy. Do the same thing for other 3. Give em a minute sometimes. They would like to finish their drink before they get killed. Happy Hunting!

I just wanted to add that u can unlock this achievement by sniping 5 enemies from a long distance with the precision rifle. I found so far the gas station base where u destroy ur first two tanks a good place. Stay in the hill staring down on the base and snipe. By doing this u can unlock "long distance relationship" cheevo too.

This may seem difficult at first because the AI seems to be omnipotent in this game, and always knows exactly where you are if youre within earshot.

The trick here is to kill an enemy without them recognizing where you are, and returning fire. Easily done with the Sniper rifle or "precision rifle" as the game labels it.

The way I unlocked this is find some highground overlooking one of many enemy strongholds in the game, give yourself atleast 100 yards between you and the nearest enemy, just to be safe (yardage can be measured when using the binoculars)

Go prone to eliminate rifle-sway, and hit RB to cloak, when the first enemy is in sights, hit RB to disengage cloak - fire- RB to cloak again. This will cause little to no energy to be used up by the cloak, but still keep you relatively hidden from the AI. Once the first enemy falls the rest will rush to his rescue, repeat this trick 4 more times without being spotted and the achievement is yours.

If you mess it up once, no worries, you will have PLENTY of opportunities to try again in the game, just make sure once you get that precision rifle you keep it, it is your best friend...achievement or not.