Kingdoms of Amalur: Re-Reckoning

The achievement unlocks when you kill your 50th bandit. There are different bandit groups in each region. In the Dalentarth region where you start the game, the Red Legion controls the area. Doing the Recipe for Trouble side quest given by Nanne Hanri, owner of the Golden Alchemy Age shop in Gorhart, and the Commendation faction quest given by Ost Ordura (found on the first floor of the Gorhart Inn) will pit you against enough bandits to get the achievement. These quests are available at the beginning of the game as soon as you enter Gorhart. Below, is a very short video showing the achievement unlock on the 50th bandit kill.

Bandits are fairly common in Yolvan (the first area), and often found in ruins and caves throughout Amalur (but so are Boggerts and other critters).

This achievement will come naturally, but if you feel the need to farm kills, you can enter and exit various buildings in Gohart and run around the paths outside (heading towards the North) to trigger ambushes.

There is also a bandit camp to the North of Gohart that has 10 or so bandits. They should respawning every 2 or 3 in game days.