The crossbows are the weapons in the second row of the archer class. To get this achievement you will need to get 75 kills with crossbows after you have unlocked the second crossbow so that is a total of 100 kills with a crossbow. I much preferred the first crossbow due to the higher damage.
Objective matches can be good for this as opponents will be a bit more focused on the objective instead of rushing in a attacking you. Due to the slow reload time of the crossbow, choose your shots a bit more wisely and while reloading make sure you are behind cover as you are incredibly vulnerable while reloading.
Any suggestions for this solution just comment. :D