The Witcher 2: Assassins of Kings

I got it while grinding for the Miser achievement. I found this on another Achievements forum, and it works well, even on Dark difficulty (depending on your equipment, I suppose). If you're on a lesser difficulty, it should be no problem regardless of equipment.

Note: At this point, I was level 17, had the Armor of Ban Ard equipped, and was using a Robust Red Meteorite Sword (silver) enhanced with a Fire rune. Playing on Dark difficulty.

When you get the "Visionary" quest (Roche's path -- not sure if the same is true for the Iorveth path, anybody know?) you'll come to the Visionary's cabin. Four harpies will attack. Kill them, go back up the path (or into the candle circle) and repeat. They keep respawning. If you're getting through them quickly, it shouldn't take long. This unlocked for me after just a few minutes. Apparently I had done quite a bit of fighting leading up to that point anyway.

This is also a good way to get the Miser achievement. Just sell the Harpy loot once you stock up.

The Visionary cabin is a good place to farm Harpy feather's but being that only 4 Harpy's spawn at a time once you enter the cabin and talk to the Visionary I found that it was far easier to progress just a little farther in the quest. SPOILER ALERT you will have to cross the Mist to get the Spear to reverse the curse on Henslet. When crossing the Mist there are endless Foes to battle that constantly re-spawn. After about 10-15 minutes of fighting in the Mist the achievement popped. All of this is subjective to how many Foes you have already killed but regardless on this battlefield in the Mist there are plenty of Foes and they are relatively easy. I think it's much quicker to face hundreds of Foes in the Mist as opposed to fighting 4 Harpy's at a time by the Visionary cabin.