This is for slowing three or more people with Fafnir's first ability. The way the ability works is you throw an axe at an enemy. The enemy hit by your axe will be stunned; enemies in the vicinity will be slowed.
If you're looking for an easy spot to do this, I just hid near the enemy buff camps at the beginning of an Arena game and did it when they were all bunched up killing the purple buff. You can see a clip of me getting the achievement below (the award pops up in the bottom-right corner).