Sinspawn Gui is the result of the Sinspawn gathered for Operation Mi'ihen fusing into a monster that breaks out of its cage and fights the party in Final Fantasy X, just as Sin approaches the shore. The party is scattered by Sin's attack, and Seymour Guado grapples with the reanimated body of Sinspawn Gui alone until he is joined by Yuna and Auron; both encounters count as one boss.
Sinspawn Gui is made up of four parts: the body, the head, and two arms. The body and the head act independently. The arms do not act, but will regenerate after the body gets either three or four turns with both arms dead.
The head, in both battles, follows a set pattern. It will use Thunder, and on its next turn start "moving suspiciously", followed by using Venom on the turn after that (which causes it to stop moving). It then repeats this pattern. If the head is attacked between when it starts moving and when it uses Venom, it will stop moving, and will not use Venom, but will restart the pattern starting from Thunder on its next turn.
The arms do not attack, but they shield the body from physical attacks. When shielding, the arms count as Sinspawn Gui having 100 defense, regardless of whether there is one or two of them present. They do not protect the head.
The body follows a pattern of alternating between physical attacks and Demi. There are some special cases, however. On its first turn in the first battle Sinspawn Gui will use a physical attack, but this does not count as part of the alternation; it will begin the alternation (starting with another physical attack) from the second turn. It may skip the physical attack and go straight to using Demi when the head is dead and the body is below 4,000 HP.
Though the first form gives no spoils immediately, Overkill still matters as item drops will be rewarded after the second battle. The rapidly proliferating potential supply of Lv. 1 Key Spheres at this point makes it possible for Steal to be accessible on the Sphere Grid, but stealing from Sinspawn Gui will yield only Potions.
A few party member Overdrives and both aeon Overdrives will see good use here against all four available targets simultaneously. Tidus casting Haste on himself and then other party members will maximize the ratio between party actions and those of Sinspawn Gui. Auron's and Kimahri's attacks are effective against the armored arms, and killing them enables further physical attacks against the body. The arms will regenerate, but count as separate enemies for the purposes of awarding AP; each kill will add its AP to the total.
Once the head begins moving (if the player has not already killed it), Wakka, and Lulu will be alerted. If left alone, the head will use Venom on a character, but Lulu's spells, Wakka's attacks, or Lancet will cancel the poison attack, or the Poison can be negated by Esuna for the same action as cancelling the attack, which frees up magic attacks for killing the body.
Second tier spells, such as Fira, should be strong enough to kill the arms in one shot, or can be used on the body to bypass the arms or eliminate the head and its poison threat.
Only Yuna, Auron, and Seymour Guado will be available. Seymour is equipped with the second-level Black Magic spells; he is much more powerful than Lulu or Yuna and can easily defeat the monster on his own. Waiting for Seymour's Requiem Overdrive to charge will take some time, but this is the only opportunity to see it.
Sinspawn Gui does not have an extra attack at the start; it begins the alternation from its first turn. After a certain point the body is not guaranteed to use a physical attack; it may skip that step and go straight to the next Demi. This happens as soon as the body has taken any damage, even if the head is still alive.
The most common strategy is to have Yuna keep the party healthy while Auron and Seymour attack. Seymour has Sensor, and thus the player can ensure he gets the killing blow with a tier 2 Black Magic. This will Overkill Sinspawn Gui and double the number of Lv. 1 Key Spheres awarded; six rather than three.