1.) In the Arena there will be a red portal across from where you originally spawn into the map. Have all players interact with it in order to be teleported to the Boss Fight.
Waves of Tigers and numerous Gladiator zombies will come at you. You must kill them.
After all the Gladiators and Tigers have been defeated the Fury Elephant will come into the map out of a huge gate. Shoot the side armor off of Fury in order to destroy it. When her armor is destroyed you must shoot Fury in the face until it dies.
After Fury is defeated you will hear numerous audio quotes. After some time the black Wrath Elephant will spawn out of the other huge gate. Kill Wrath the same way in order to complete the boss fight and have the cutscene play.