The actual description for unlocking the achievement is as follows:
As Loki get the killing blow on an enemy with your Decoy.
Loki's second ability (
or 2) is a stationary decoy with a circular explosion radius around it that does base damage 75/125/175/225/275 (+100% of your Physical Power). You cannot explode the decoy early so you will need to wait the 3 seconds for the decoy to explode. Many times people underestimate the explosion power of a decoy, or do not realize it damages them (thinking it only damages minions).
The best way to snag this is to catch someone with low health when they are trying to clear a camp or defend a tower.