All Credit to Rallos achievement:
You need to get all A restless guardian of Zek down to 25% or lower health. When Avatar rages, he will kill them. An animated axe does not count towards this achieve, so you can kill them yourself. When it gets tricky is when boss gets to 65%. Best thing to do is do no damage tanking, and make sure you don't have a DS as the new DS do a lot of damage. Get that last A restless guardian of Zek down to 25, then wait for boss to rage, then dps boss down to 60 to force port.
Once boss ports, you can burn him down so long as you have a strong tank that can survive 8 mobs, and keep all aggro.