Reaching Ultima Weapon
After defeating Bahamut in the Deep Sea Research Center, the player needs to return to the Ragnarok, talk to a party member, and return to the island. A large vine structure will have grown in the place where Bahamut was, and the player can use it to descend to the lower levels.
The lower levels are reached by expending units of steam pressure to open doors. The player begins with twenty units, and must conserve ten for the bottom to fight Ultima Weapon. In the third floor the player can open a way to the Steam Room to gain extra units of pressure. With the extra units the final door can be opened, leaving the player with exactly ten units needed to fight Ultima Weapon, and the excavation area can be traversed without random encounters if the Enc-None ability is used. If the player does not gain the extra units, they can ask Zell (if he is in the party) to open the final door without using up any steam pressure units, and the player must traverse the excavation area facing many challenging fiends in fixed battles that cannot be avoided with Enc-None.
During the battle the player can draw the game's most powerful Guardian Force, Eden, from Ultima Weapon. Ultima Weapon won't reveal its HP when scanned and the battle's difficulty is determined by the level the party is when facing him.
Ultima Weapon's most dangerous attack is Light Pillar, which always inflicts 9,999 damage to a single character. It can use it multiple times in a row. Ultima Weapon will also cast Quake, which can be absorbed with correct junctions. Gravija drains 75% of the party's current HP and cannot kill. Meteor hits random targets 10 times. It will also attack physically from time to time.
If the player can gain a sufficient advantage, Ultima Weapon is a useful place to draw several lots of Ultima spells, which are great for junctioning. Ultima Weapon and Omega Weapon are the only enemies that have Ultima in their draw-list. If the player opts not to mug Ultima Weapon, they win 100 Ultima Stones after battle, which can be converted to magic. If the player mugs they get Three Stars instead, which is used to teach Expend3-1 to a GF, or refined into 100 Triples.
A stock of Full-life and Life spells, and/or the Revive command, are useful, though the player can opt for using Phoenix Downs to revive allies in low health to use Limit Breaks without having to cast Aura.
As Light Pillar will kill a character no matter what health they are at, there is no need to configure the party's HP to the max; instead, one can use the freed slots and magic to boost other stats. Speed is of essence, as Ultima Weapon can cast Light Pillar many times in a row, occasionally with no time to recover in-between.
The party takes a risk if summoning GF on Ultima Weapon, as it will immediately cast Gravija. The party should start with either Meltdown or Doomtrain to apply the Vit 0 status, greatly increasing the damage done by all attacks, followed up by casting Aura on Squall, and using Renzokuken. Depending on his junctions, the model gunblade in use, and the final attack of the Limit Break, this strategy can take down Ultima Weapon in only a couple of attacks, or even one, should Squall use Lion Heart.
Alternatively, Rinoa can be used with Angel Wing by removing all offensive magic save Meteor (this includes status magic) and boosting her stats with other types of magic: Cure, Cura, Curaga, Regen, Esuna, Haste, Aura, Triple, Double, Protect, Reflect, Shell, Life and Full-life, are the options for junctioning. Rinoa will chain-cast Meteor, at 255 magic with Angel Wing and Vit 0 on target this can do upwards of 40k per turn (6–7k per meteorite).
With Auto-Haste and a high enough Speed stat (junctioning Triple to Speed) she can lay waste to Ultima Weapon before he can kill anyone. Angel Wing does not use up the magic it casts, and is the second best method of dealing damage in the game with Zell's Duel being the best using the "Armageddon Fist" tactic.
Ultima Weapon is a Level 7 Boss Card in Triple Triad used for playing the minigame and for turning into an Ultima Stone with Quezacotl's Card Mod. It is a rare outcome of successfully using the Card command on an Elastoid or a Tri-Face.
Behind the scenes
The sword Ultima Weapon carries is Cloud Strife's Ultima Weapon from Final Fantasy VII. This is due to the monster being based on the Ultimate Weapon boss, who would drop Cloud's best weapon if defeated.
Ultima Weapon is the second enemy that awards a SeeD rank when defeated, the first being X-ATM092.
Ultima means "the last" in Latin and many Latin-based languages. In the Spanish versions of many Final Fantasy games, the spell Ultima's name is written as "Artema", a close transliteration of the Japanese katakana.