I have seen a lot of people struggling to complete this task with the limited about of sniper scavs. I got this done in 5 quick raids so I will share the easy way I done it.
My method was to take whatever bolt action you want and go to woods at night. Scale the mountain and pop the sniper scav up there. He will almost never be aware of you so it’s an easy kill. The fog/rain/darkness will stop people easily killing him from a distance(which was also a real pain in the fucking arse as I needed some PMC kills and couldn’t see shit). Also I only ran into one other player that was a playerscav. I imagine everyone loading in is just doing loot runs in the dark staying away. Plus sometimes there is an easy shturman kill up for grabs.
Got this done in 5 raids, took less than an hour but you could do it quicker if you don’t loot afterwards.