The requirements for this are by using Cernunnos to "kill an enemy god who is poly-morphed from your Wild Hunt Ability." The "Wild Hunt" ability is Cernunnos' ultimate ability that is typically unlocked at hitting level 5 in a match. As with most achievements on Smite, its all based around team comp on both sides of the game to determine whether or not you can pull off the stunts necessary to get the character specific achievements. Cernunnos is a hunter, so a simple hunter build based around attack speed and penetration would be good for this and just him in general.
This video by YouTuber "SuperTeeds," should better help you understand the character and how they work.