This boss is weakness to Fire & Holy, heck of it even Ultima if you learned it! Owzer's Mansion is full of rare loots; if you brought Locke with you make sure you steal as much of those Moogle, Nutkin, Tabby, and Chocobo Suits, and Super Ball before facing Chadarnook.
Chadarnook (Starlet)
Level: 37, HP: 56000, MP: 9400
Chadarnook (Demon)
Level: 41, HP: 30000, MP: 7600
Steal: None