I found Devil's Pit to be the best place to do this. Warp there and head down and to the right and there is a wooden platform. Lure enemies there and hit them once to weaken and then position them facing the right direction and blast them off them platform. I was able to get about 3 per trip so then you must warp out of Velen and then back and they will respawn. Took me 15-20 minutes but it works. Enemies with axes don't fly as far, concentrate on the ones with the billy clubs.