Knock a Zandalari Beastcaller from his flying mount.
This achievement is a bit odd. There are http://www.wowdb.com/npcs/69780-zandalari-beastcallers riding http://www.wowdb.com/npcs/69156-zandalari-skyscreamers all over the beachfront south of Za'tual. However, while hitting the Skyscreamer will knock the Beastcaller from his mount, it will not give you achievement credit. Instead, you have to hit the Zandalari Beastcaller himself with an attack when he is mounted, then you will get the achievement.