There is an excellent spot for this achievement. It is located south of Garin Estate at Martin Fueille’s Farmstead in Velen. Look East of Novigrad (Oxenfurt Gate for a little more precision). There will be around 8+ cows on the grass and you can make them respawn by meditating. Nice place to farm raw meat and milk, too. Here is a video to help:
credit goes to Guides Gamepressure for the video.
EDIT: A1an006 in the comments mentioned that:
it only seems to work with swords or crossbow. I tried it first with Igni, but it didn't work. Tried again with swords and the achievement popped.
So if that's helpful to you, do show some appreciation in the comments. Thanks A1an006!