In my second game I happened upon the place coming from the east. From the cliffs I was able to snipe quite a few of them without VATS. In sneak mode with a silencer they seemed to not be able to zero in on my location so the picken was fairly easy. After killing all I could see I moved in a little closer and took the rest. If you can get close enough to that hopper thats up against the cliffs it's a great sniper position. If you want to try another trick, use a stealth boy and leave a trail of mines going up the ramps to the hopper then get their attention. Any time I'm going strictly for head shots I use hollow points.
Anti material with incendiary work great, too. If your sneak and crit is high you can pick them off very easily as it often takes 3 good shots to dropem.
They can def one hit you if you get flanked. I can't even use VATS with sniping rifles as the accuracy always svcks opposed to hard scoping. It's a shame the AI is SO stupid, they never path for you if you're far enough away.