Simply load up campaign mission 3 "No Man's Land" and proceed through the level till you are prompted to take control of Riley.
Kill the first enemy as instructed and then sprint to the second enemy to attack him and beat Hesh to the kill. For the next group of 3 outsde, wait for Hesh to take out the first enemy then kill the solider first as instructed, followed by the scientist.
Proceed through the rest of the level until you are prompted to once again control Riley. Follow Hesh's orders and traverse carefully through the grass and a few enemies will be lined up easily for you to kill. Need be you can run out and get a kill or two before being spotted if you are close to your 10 kills but know that they will gun you down fast regardless of what difficulty you are playing on. Note there is more than enough enemies in the second sequence for the achievement.
I created this video to help:
Hope this helps.