Deifer the Untamed circles very quickly and even on a land mount you can't keep up with it for long. It can be attacked, but it doesn't attack back. If you get close enough, you're able to mount Deifer and gain access to two special abilities, Harry and Stunning Strike. When mounted, you'll continue to circle the Banks of Life and the other wildlife there won't hesitate to attack you.
The first ability, Harry, causes some slight damage and slows the beast. The slow effect stacks up to a maximum of 50%. If you only used Harry repeatedly on Deifer the Untamed, then it would take a very long time to kill it. The damage taken from the other creatures while mounted will kill you faster as seen in the posted video by Sipder2.
The second ability, Stunning Strike, stuns the beast and forces you to dismount. You have 10 seconds to damage Deifer before it continues it's circuit.
If you're trying to kill it solo, then one strategy is to clear the hostile wildlife in one area nearby the beast's path. I found the southern part of the path to have the least number of mobs to kill. Give yourself enough room to successfully mount, slow the beast (if needed), and then stun it. Do as much damage to Deifer as you can before it starts running again. Wait again for Deifer to come around and repeat. It took a few minutes to kill it this way, but it seemed to work the best for me.
You're rewarded a Gorm Needler upon a successful kill.