In order to earn this achievement you need to kill someone with Athena's ultimate (
or ability 4).
Athena's ultimate is a teleport to any teammate on the map and upon reaching them, an explosion of damage in a circular radius around the targeted teammate. The ability itself does a lot of damage, and late game can do up to a quarter or more of an opponents health depending on their class and build.
One of the easiest ways to secure this achievement is to attempt it in arena and wait for a teammate to dive (go in very hard) for a weak opponent. This tactic is all-to-common in arena and can actually be a simplistic set-up requiring no communication or additional work on your part. Simply wait for an enemy to be weak enough that your teammates decide to dive, target the chasing teammate and activate the ultimate.