Can easily be done at the "Hanged Man's Tree" fast travel point in Velen (middle of the map). By the time you enter Velen you will automatically have a crossbow in your inventory. Equip the crossbow and Igni sign. Now kill some wolves near the fast travel point with the Igni sign, then use your sword until only one is left. Kill the last one with your crossbow (press R1 on PS4 / RB on Xbox to shoot).