Normal: "A fiend that protects the sphere in the Gagazet ruins. He halts his prey with a sticky web before attacking. Maybe he's as scared as you..." Oversoul: "A fiend living on Mt. Gagazet. He halts his prey with a sticky web before attacking. During battle, he grows progressively stronger, a vile breach of etiquette." — Scan
It is best to have two party members in attacking-type dresspheres (such as the Gunner and Warrior) and the third as a Songstress. The Songstress should continually use Darkness Dance while the other two attack. If a Warrior knows the abilities Flametongue or Liquid Steel, those could also be used to inflict damage.
In the Fiend Tale endings, a notable Boris was a trickster who messed with the Gullwings' heads by claiming to be attacked by various fiends just to get Shinra's attention. On the day he was let go, he did get attacked by a Behemoth and was saved by Shinra.