Call of Duty: Modern Warfare 2
51 Achievements
27-33h
Ghost
Plant the C4 in 'Cliffhanger' without alerting or injuring anyone in the blizzard.
3.1%
How to unlock the Ghost achievement in Call of Duty: Modern Warfare 2 - Definitive Guide
This trophy is easy once you know the right route to take and where the enemies are. You can't get spotted at all or the trophy won't pop, you know if you've been spotted because the enemies stance change and they walk around with their gun pointed forward or MacTavish says "You've been spotted". The best way to do this is just follow the video below exactly, don't shoot the enemies or the trophy won't unlock, just wait for MacTavish to snipe them. Also don't run, just prone or crouch and move slowly. Upon planting the C4, the trophy will pop shortly afterwards. Video by Nextgentactics:
https://www.youtube.com/watch?v=_Zpl8pJOKS0
This can be done in any difficulty. After you climb the ice and kill both sets of enemies, you and Soap will separate and a blizzard will appear. Take out the gun with the heartbeat sensor and crouch towards the objective. Do not kill or injure anyone and make sure you aren't caught. If you are caught, you can restart from the last check point. Wait for the guard near the objective to move near the car and pass next to him. Do not go inside the building. Move towards the other building on your left, and head towards the edge of it (near the small hill). Soap should mention something about a car but just keep going. There should be one or two dots in your sensor but they can't see you. Ignore Soap when he says there are enemies on the runway, just keep going and plant the C4.
Another way to do this is to follow this video by Mistical: