"As Tyr, kill 15 enemy Gods with the damage from Lawbringer".
Lawbringer is Tyr's ultimate (fourth ability). It does damage when landing after the leap. It can be difficult to hit sometimes, but can be used over walls which can help surprise the enemy. Building damage and penetration items will increase the damage it deals, increasing your chances of a kill. Use it on lower health enemies as it will never 1-shot anyone, even weaker gods.
Edit: This achievement seems to be bugged, as although it doesn't say it, this needs to be done in one game. This makes it considerably harder. Even with maximum cooldown reduction and damage you will struggle to get this legitimately.