You'll notice that the Ancient Fiend has two Hardened enemies to protect him while he chills on the Back Row; you may be tempted to try and take them out first. Do NOT fall for this trap. They'll take a while to kill, and meanwhile the Ancient Fiend can hit HARD. Shadowus will do 500-700 damage to everyone, most likely, and his Successive waves will do 700-1000 as well. Not only that, between Shuffle and Bind Hit, your party's formation will be pretty screwed up and inefficient.