The light weapons are the weapons in the second row of secondary weapons in the archers class. To get this achievement you will need to get 75 kills with light weapons after you have unlocked the second light weapon, so that is 100 kills total with light weapons for this achievement.
The light weapons unsurprisingly fast, use it to your advantage. The two swing combo (by pressing swing again timed against your first) works well with these types of weapons. However they barely damage knights so unless you are good at blocking since you will be in the archer class.
Any suggestions for this solution just comment. :D