This solution can be used at any time in the game, has no prerequisites, and can be completed in about 5-10 minutes.
Head over to Paya Luas Military Airport in the northeast of the map - coordinates are X: 28412 Y: 5704
Lined up next to the runway that runs west-east (horizontal) there are three hangars. The middle hangar contains a Si-47 Leopard military jet, and just outside there is a URGA-9380 heavy duty truck. Get in the truck.
There should at this point already be two guards near the truck shooting at you. Head down the runway a bit but not too far - if heading west don't go past the helipad area, and if heading east you don't have to go beyond where the runways intercept.
Turn around and head back towards the middle hangar where you got the truck. There should be a few more guards now, all running towards you. Go ahead and run them down (shouldn't be too hard, they're usually in a straight line for me) as you pass right by the hangar.
Once again, don't go too far. Make a quick U-turn and head right back at the hangar. Yet again, there should be even more soldiers running towards you in a group just begging to be run over. Continue going back and forth like this and you should have this cheevo in no time. My average grouping was 4-5 guards and I usually killed 3-4 per pass, easily accumulating 30 kills in under five minutes.
If for some reason you cannot get the guards to keep spawning, follow this guide right up to stealing the same truck. This time instead head west down the runway to the helipad area.
As long as you have no already killed him, there will be a colonel near this helipad right out in the open, along with four elite guards. Go by him slowly at first to get his attention, then make a U-turn shortly down the runway. Upon heading back towards his position, they should all be running at you. Try to hit them at an angle so that you kill some of the guards, but not the colonel.
As long as you do not kill the colonel, you can drive all the way east down the runway, turn around, head back, and his guards will be completely respawned. Using this method you can even hit a few guards on the way back. Keep the colonel alive and kill a few of his guards each time and you should still have this cheevo in under ten minutes.