There's a gang spawn in Pacifica which can be easily farmed. It's something like 5-7 NPCs, standing around some vehicles, and a single round in the right location from the projectile launcher can trigger all the vehicles to explode, killing all of the gang members.

If you do it from the right spot (the top of a nearby building which you can't climb), then you can position yourself at a point where the difference between crouching and standing pushes you past the rendering distance of the spawn.

Crouch, kill, stand, repeat.

I did it from the penthouse floor, not the roof, and had to strafe back and forth instead of crouching/standing. From the roof, you can probably go 15-50 in under an hour. I suggest level 15, because below that and the projectile launcher doesn't reliably kill the targets, even with the damage from the exploding vehicles. This is also on Very Hard difficulty.