In order to do this you need to unlock the Puppet ability, found in the second row of the Axii magic ability. This allows you to temporarily turn an enemy into an ally. You can level this up a few times to make your puppet more powerful. In order to trigger it, you have to hold down the trigger (rather than tap), otherwise you cast the normal 1st level stun.
This works best when the enemy your targeting is close to others. If you use Puppet on one whose off by himself he'll over just stand there and not attack anyone else.
Beyond getting the achievement itself (which isn't too hard), this ability is extremely useful for getting through a lot of battles, particularly earlier in the game (when you're weaker) on harder difficulty levels.