The Devil’s Pit in Velen is one of the best Places to get this Achievement. The Bandits will respawn again if you leave the Map (Fast Travel to Skellige, Palace or White Orchard) even if you looted the Place completely.
Kill the Bandits that are carrying two-handed weapon, for some reason that won’t fly as good as the ones holding only one-handed weapons go to the Place in the Screenshot below and use LT to block and get them near the edge, now use the Fus Ro Dah (Aard Sign) to knocking them off somewhere high.
Having Tawny Owl to faster regenerate Adrenaline helps.
