In this quest you have to kill any 30 hostiles from a distance of over 100m with AR-15 platform weapons. You can use any of the following:
A recommendable spot to shoot from would be dome area on Reserve as you can get a good view of the Scav spawns around the map.
It is recommended to use a suppressor as shooting loud from this position can attract unwanted attention.
AI Scavs tend to search and loot dead bodies. For a brief moment they will be stationary whilst looting, allowing for an easy headshot kill. Killing targets out in the open will leave their bodies as bait allowing for AI or players to loot them letting you gain more kills and establishing areas of interest to shoot at.
Use a high zoom variable scope for better accuracy and non-tracer ammunition for better concealment.