All the enemies you encounter prior to putting on the ski goggles can be eliminated.
After you’ve put on the ski goggles Soap will go up the cliff. Keep an eye on your heartbeat sensor (Baby Monitor) to keep track of the enemies. You need to crouch walk your way to the car that’s parked on the opposite side of the road from where you are. There are
two enemies in this area that Soap will need to take out. First one will be behind the car, the second one will be walking to the left side of the road from the small building.
Once Soap has taken them out, stick to the left side and walk up the small set of stairs and you’ll see some boxes in front of you. Crouch next to the boxes until the
vehicle has gone past. Once it’s gone, turn left and stick to the right side of the road whilst crouching. Then turn right once you’re past the building.
The last enemy will be on the hill in front of you, let Soap take him out. Then crouch walk your way to the C4 placement.
You will have done it correctly if Soap says "No kills, no alerts, impressive Roach."
If you're still struggling, feel free to use this as a reference.
Hope this helped.