I was done with all the quests in Investigations and needed 10+ more Ranger kills.
The place that worked for me was A/C Systems. Started at Eagle Limited AWE cp, jumped up through the fan right next to it that leads to the Service Vents and went out on the bridge in A/C Systems where you have the power cores that open some locked doors. 4 of the 5 times I was there Rangers spawned in 2 waves.
First off I tried doing at Bright Falls AWE where you fight Hartmann (Bright Falls AWE Entrance cp) but was unable to spawn any in 15+ attempts. Have seen others having success here, so if A/C does not work you can try here.