You'll be killing a lot of these if you use the power leveling tip I provided in the guide for:
Short answer is that these have a high spawn rate in Arba on the Arba Town - Hero map. Check these 4 zoom points for spawns:
Northgate Street
Northern Arba
Parade Ground
Main StreetTrophy may be glitched as I had killed 5 before it decided to pop.