The crossbows are the weapons in the second row of the archer class. To get this achievement you will need to get 25 kills with the first crossbow.
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