IXEpIcRaNdOmXI's strategy is the best, especially if you missed out on your first play through (like me!) and want to get the remaining kills you need. So, to piggy back on EpicRandom's, the earliest opportunity is in the tutorial after the very first red emergency box; in it should be a flare gun with 3 flares.
Drop down and use it on the 3 Taken, then go forward a few feet til the one crawls over the rock. You can take him out now or run back to trigger another enemy (maybe 2 more? I can't remember), then hang around in the grassy area until another group of Hitchhikers come, take THEM out, then run off the cliff face and die, reload the checkpoint and do it again.