"As Yemoja, deal damage to 3 or more enemies in a singe use of River's Rebuke."
River's Rebuke is Yemoja's ultimate ability. It summons two massive walls of water vertically in front of you (so it blocks all movement to the left and right) that blocks enemy movement and projectiles.
It sits there for a few seconds before crashing inwards and damaging, slowing and trembling all those it by the walls. You simply need to damage three or more people in one use.
Probably best attempted in late-game Conquest, as you can use it to block grouped up enemies in a lane so they can't escape out of either end, Also possible in Assault or Arena, although will likely be a little harder unless you have some help trapping enemies in each end of your ultimate.
You could pair with the usual Ares, Cerberus or Da-Ji to pull enemies in using their ultimates, or with Ymir and his wall ability, potentially even Odin if he can trap enough enemies in his cage.