There are a total of 25 enemies to kill on the level total. The groups of enemies can be split into 3 groups:
The Traincar enemies (8 total)
The group of Enemies on the train from your car with Khan (5 total)
The group of enemies while you are on the train. (12 total)
The first group- The first group is the hardest group. The first two cars have 4 enemies on them. Aim offset to ensure your bullets make contact. If you do not kill these enemies fast enough, Khan will destroy the cars himself. If this happens, restart the checkpoint. After killing the first 4, there will be a small cutscene of a Khan destroying a car in front of you. 2 more cars will arrive with 4 more enemies. Aim offset to ensure your bullets make contact.
The second group- The second group is a bit easier to kill, but will still need to act quick before Khan tells you to jump onto the train. As you come right behind the train, kill the Machine gunner. When Khan brings you parallel to the train, kill all 4 of the enemies shooting you. Again, aim offset so your bullets make contact.
The third group- The third and final group is the easiest group to kill, as they are all in the narrow train cars. There are 12 enemies here, and you can kill these guys however your wish. The achievement should unlock as you kill the final enemy.