This boss' arms will protect the body against any physical attacks, using Auron and Kimahri to quickly destroy them is the first priority. Once the head begins moving (if you have not already killed it), Wakka, and Lulu will be alerted to the danger. If left alone, the head will release a Poison attack on one character. Have Lulu cast Fira on the head, as Sinspawn Gui is rather weak to fire. The arms will regenerate a few rounds after defeat, but the more times they are killed means the more AP is awarded. Following the battle a cut-scene showing the disastrous consequences of the operation will be shown and the party will be separated.
Gui will regenerate around half his original HP and stats. For this battle, only Yuna, Auron, and Seymour will be available. Seymour is equipped with the second-level Black Magic spells and is much more powerful than Lulu or Yuna at this point in the game and can easily defeat the monster on his own. The most common strategy is to have Yuna keep her teammates healthy, while Auron and Seymour attack. As this is the only time Seymour is available in a battle scenario, it is also the only chance to see his Overdrive, Requiem. Unfortunately, it takes quite a long time to charge, but the wait is worth it for the one-time-only experience.