I recommend for this achievement the easy difficulty because after you trigger the point where the achievement starts a lot of new enemies spawn on the map inclusive 4 new snipers and new vehicles.
The achievement starts after you sabotage the reactor in the bunker. It is the last primary objective on the map before you can leave the map. I run in my video on the left side of the map through the cave, so clear this route or any route you want before you solve the last primary objective.
After you do the primary objective (Sabotage the Reactor) run without shooting (
) to the exit point of the mission. Melee kills (
) are allowed.