Lady Naz'jar will occansionaly summon a geyser on the ground under a random player that erupts after 2 seconds, launching the target into the air. The spell deals 23,750 to 26,250 damage to any characters standing over it when it erupts.
When she casts Waterspout at 66% and 33%, you'll be met with two Naz'jar Tempest Witches and one Naz'jar Honor Guard. These are the minions that the achievement references.
The requirement for this achievement is to have Lady Naz'jar kill either her Naz'jar Tempest Witch or Honor Guard with her Geyser ability. Where normally, you would either kill or CC these adds, you will have to burn one of adds down until it can be killed by the Geyser and then position it where a Geyser will erupt.
When the adds come at either 66% or 33%, kill two of the three adds. Then burn the remaining add's health down low enough to be killed by the geyser. The honor guards have 387,450 health. The tempest witches have 232,470 health. CC the mob on top of the white zone that indicates a Geyser will erupt there.