This member of the Warring Triad can use a basic attack, Fiendish Rage (which is four times as powerful), Blizzaga, Northern Cross, Absolute Zero, Force Field , Reverse Polarity, and Targeting. Force Field you are not familiar with - it randomly nullifies an element, and is a Lore Strago should get. Targeting simply tells you who will be hit by Fiendish Rage.
Begin with Slow on the enemy, then self-Hastega and Mighty Guard are quite helpful. Offense should consist of Lv. ? Holy, Holy itself, Flare, Meteor, or Ultima. Golem may be helpful, as could Alexander and Bahamut. Also try to note that you will want to cast Dispel when the message about Fiend’s aura shaking appears.
Two down, one to go. Use the Save Point nearby, then move into the pipe behind where Fiend was. Get on the switch within, then swap teams to…
Assuming you haven’t moved, you’re still on the switch. Go south and out the door. Go east and inside, then go north and through another door. Go north again and through another door. You’ll find the final statue. Heal and equip anti-Zombie Relics. Once near the statue, the fight to end the Warring Triad – or you – will begin.