The crossbows are the weapons in the third row of the archer class. To get this achievement you will need to get 25 kills with a javelin. Javelins work much like a spear, however your slash attack will launch the javelin. You can kill with the throw attack, stab attack or even overhead (which actually uses a shield, but it still counts.)
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.
Any suggestions for this solution just comment. :D