I highly recommend equipping the accessory Gaze of Despair. This can be achieved anywhere but I recommend any fight featuring large crowds of regular, small enemies. Chapter 2 is great, as is the out of body battle in chapter 6, verse 6 (where you can also earn the achievement 'A Mother's Love.') The prologue is fine too but because the enemies won't drop their OP weapons after torture attacks, you'll have to rely on your own skills.
The easiest way to wipe out many enemies at once is to torture attack an enemy with a polearm. Pick up the weapon after they drop it, get into the middle of a crowd, activate witch time then press B. Spin around as fast as you can! If you don't kill enough enemies for the achievement, you will certainly refill your magic enough to perform another torture attack straight away and repeat the process. This is also a fantastic way to earn high scores and crazy amounts of halos.