As Artemis your ultimate/4 is a AOE from the point you're standing when you activate it.
You summon an animal that will bounce around in that AOE to each enemy dealing damage and stunning them for a brief moment.
Getting this achievement is super easy, just run up to a group of enemies and activate it. During a team fight this is useful anyway, you should get this done like five times over in a single match but you only need to do it once.