Ultima Buster is a special fiend from Final Fantasy X. It is bred in the Monster Arena as part of the Original section, and is available to fight after five of each fiend roaming throughout Spira have been captured.
Ultima Buster has two arms that shield the body against physical attacks, and its head will release its Contamination attack, which inflicts Poison (always), Zombie (always), Confuse, Full Break and Slow. The arms and head each have 80,000 HP of their own. The arms will regenerate after the body gets either three or four turns with both arms dead.
If the head is attacked between when it starts moving and when it uses Contamination, it will stop moving, and will not use Contamination.
The arms do not attack, but they shield the body from physical attacks. When shielding, the arms count as Ultima Buster having 255 defense and gains Armor status, regardless of whether there is one or two of them present. They do not protect the head.
It uses physical attacks and Ultima, a powerful dark magic spell which damages the entire party.
It drops armor with the Break MP Limit ability, and weapons with the Overdrive → AP, Triple AP, and Triple Overdrive abilities; useful for doing the AP trick with Don Tonberry. Its equipment drops provide a 6% critical hit chance bonus instead of the usual 3% (this is relevant to weapons and armor, but only to attacks that deal physical damage).
Auto-Haste and Auto-Protect for all members helps. A party member should cast Ultima to damage both the head and arms. Yuna should summon the Magus Sisters and have Mindy use Passado. Wakka with his World Champion maxed should use Attack Reels and get x2 hit slots. Everyone can Entrust their Overdrives to Wakka to make short work of this battle.
Another tactic for quick victory is to attack the arms and spare the head. All participating members should have maxed out Strength, Defense, and Magic Defense (in case Ultima Buster uses Ultima), then use Quick Hits to assure that Ultima Buster will retaliate in small chances.
Ultima is Latin, and in many of today's Latin-based languages, for "The Last", in its feminine sense. However in the Spanish versions of many Final Fantasy games the spell name is "Artema", a closer transliteration of the original Japanese katakana.