This easiest way to get this done post-campaign is using Mission Select: Chapter 2 Mission 14.
Purchase the Delta Force specialist pre-mission for 70,000 command points.
When the mission starts position your troops near a building to the east such that the Delta force member is in front and the two tiny troopers are behind him. There are 3 green foot soldiers south of your starting location that should see your troops and proceed to kill you tiny troopers, leaving only your Delta Force member.
There is one health pack in this mission, so I would recommend spending 15,000 command points on more health packs if your health is running low.
There are easily 25 enemies in the level so it will take 2 runs to get this achievement. I do not believe air strike/missile/grenades count towards the kills so only use your regular weapon.
My favorite grinding spot for command points was Chapter 1: Mission 2. On three stars I was able to run through the mission in about 1 minute collecting the 2 dogs tags and using the exploding barrels to quickly take out enemies. This earns a little over 6,000 command points per run.
Thanks to Reabo for posting a great video