This will likely be your first achievement. Soon after starting the game, a farmer will ask you to save his horse from its attackers. Kill these 2 Satyrs and 1 Ravenous Boar.
Then go to Helos village. If you're quick, you can kill an enemy or two at the entrance. Once you talk to Diomedes, go across the bridge to the north. You'll find plenty of Ravenous Boars, Carrion Crows, and Satyrs to kill.
The achievement pops soon after your 10th kill.