To defeat Araxia you need to enter the final Monster Gate, where a cutscene will ensue. You will fight him alongside Big Bone (they both have 4000 Hp), dont worry though, you only have to hurt Araxia. After dealing roughly 1000 damage, Araxia will jump into the air. You need to attack him 3 times so that you force him into Big Bones. This will knock Big Bones down again, stopping his attacks against you, which hurt ALOT. Rinse and repeat. This boss fight is rather tough, unless you are either lucky of who Araxia attacks, or have the correct set-up.
Reynold: Robot or Pumpkin, equipped with Skeleton of the Wolf. Keep reynold as the main damage dealer, doing 400 - 500 per hit.
Everett: Yeti equipped with either Disembodied Six-Fingered Hand, or Galvanized Mummy Gauze, as you want to keep Everett alive so that he can protect the team with his Yeti's Abominable Armor. You need to use this when Big Bones is upright and glowing (Pre-attack phase), the only exception is if you have the chance to knock Araxia into him, do that instead.
Wren: Unicorn equipped with Headless Vampire Bat - Wren is your healer for this, just use My Pretty Panacea on a member of the party who will die from getting hit, or either dead/incredibly injured after being attacked.