As Raijin do damage to three or more enemies with his minion Raiju.
2nd Ability - Raiju: Raijin marks a target enemy by dismissing his ally Raiju, who hides in the enemy's navel. While asleep, Raiju slows and reveals the target to Raijin's team on the minimap. The next time Raijin damages the target, Raiju awakens and claws his way out, doubling the slow and damaging the target and 5 nearby enemies every .5s for 2.5s in his attempt to flee. Subsequent enemies hit take decreased damage.
Due to the changes and patches that have been released since this god has been released. Previous solutions and guides are not working, At Least not for me and my group. We have tried numerous times, multiply sessions to get the achievement the way they described how to obtain it, in the other solution and in the walkthrough was simply not working at all. We was in joust boosting and I tried hitting all 3 gods together/separated, triggering the effect with basic attack and ability attack, Fully maxing the ranks in his second ability and even waited until level 20 to max rank everything and nothing seemed to work.
The way I obtained it was by casting his 2nd ability on a minion, damaging the minion while 3 gods was around the minion so Raiju can damages the gods.
I believe the game is not counting the god you cast the 2nd ability on regardless if they get damage by the ability or not and just counting the gods who get damage after the ability gets triggered. So if you are doing this in a 4v4/5v5 hitting 3+ plus god will require luck if not boosted.