The easiest location I found to get airborne ranger kills is the A/C systems location.
If you can get them to spawn there it spawns about 3 rangers so it will make this go a lot faster than trying to grind it out at bright fall AWE entrance or crowd control on the arcade machine.
I usually ran past the mold hiss (not kill them) and hung out at the end of the bridge until the rangers spawned. (Not sure if this is needed to get them to spawn but it's what worked for me)