I found the quickest way to do this was to farm kills on mission 3-6 Bust a Tank. This method will cost you a minimum of 18,000C a go and net you 4-5 easy air-strike kills each time.
Start the mission and collect the nearest dog tags. This will help towards the achievement Tag Collector.
Purchase a Grenade Package from HQ for 4,000C to help take care of the technical that will approach.
Purchase an air-strike Package from HQ for 18,000C and make your way left. If you stopped to pick up dog tags and purchase a grenade the technical should be reaching you just as you meet the tarmacked road. Use the grenade on the technical to soften it up and finish off with your troops.
Make your way North up the Tarmacked road and there will be a vehicle filled with troops on your right. approach carefully resisting the urge to shoot. When the first troop exists the vehicle, launch the airstrike just south west of the vehicle. Run south away from the vehicle to avoid the blast/shots. This will net you 4-5 kills each time, then from the HQ abort the mission and then repeat.