The first time you encounter enemies in the DLC (Mission: A Roll Of The Dice), there will be exactly ten and you need to kill all of them. Try to gather them up in a bunch. Call the Arrow Storm (Hold ) and you should get all ten, unlocking the achievement. If you don't you can restart the mission since it is right at the beginning of it.
Alternatively, load up Sequence 6, Memory 1, entitled "Gatekeeper." In this mission French Soldiers will rush in from three gates. Simply wait for a little bit after the mission starts for a bunch of enemies to amass. Once there are a lot of them unleash your arrow strike and you should be able to get at least ten kills.