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Delta Force is a tiny trooper equiped with three airstrikes. It is the most expensive of the specialists.
I found the easiest mission to do this on is 23: Behind Enemy Lines. This is because there are 3 spawning barracks and a number of enemies, and you are only given one tiny trooper (and thus your specialist is more likely to get the final kill).
If you walk by the barracks and then away from them, and then back again (but not within the enemy range) 3 or more should be spawned at the door, and you can kill them using the specialist.
You can use this method for any achievement based on specialist or equipment kills.