Major Numerus is a superboss in Final Fantasy X-2: International + Last Mission and the Final Fantasy X-2 HD Remaster. It is a Hydra-like fiend that consists of four parts, each with unique attacks and statistics—Unum, Duo, Tria, and Quattour—and will start out the fight with Spellspring.
Major Numerus has incredible stats, and has access to over 30 abilities, making it one of the most difficult superbosses in Final Fantasy. Major Numerus acts quickly, and will use its most devastating attacks at a whim, and Protect and Shell on all of its heads. Therefore, the player party should be equipped with Haste since the start of the battle. The player must be extremely fast and fluid to stand against Major Numerus.
Unum has 555,555 HP and the deadly Radis Command, which is a single-target, defense-ignoring physical attack with a damage constant of 58 that can deal extreme damage in the range of 26,396~29,806. Duo and Tria have 666,666 HP, with the former having Venam Command, which inflicts extreme damage to all characters (12,879~14,542 HP damage) along with Poison, and the latter having Fulmen Command, which inflicts extreme Lightning-elemental damage to the party. Quattor has 777,777 HP and can use Mortus Command, which ignores defense and deals 16,739~18,901 damage to the entire party.
Before fighting Major Numerus, the party's level should be 99, with all abilities of the Mascot dressphere mastered. Rikku must have learned Dispel from the White Mage dressphere, and Paine must have learned Nonpareil from the Samurai dressphere. The key to the battle is Miracle Drink, making the party invincible to damage. It must be cast at the very start of the battle, because the party—even well-prepared—will automatically lose.
The player must have more than one Dark Matter and mix it with a Potion to mix Miracle Drink. Invincible, or anything that grants Break Damage Limit, like the Last Resort Garment Grid, is needed. One girl must also equip the Highroad Winds Garment Grid for First Strike, and move the Dark Matter item to the top of the inventory.
At the beginning of battle, the player should quickly mix Miracle Drink. This takes a lot of luck to not have any of the girls killed while waiting for it to happen. If a girl is KO'd, the player can still win, but it will be very difficult. Afterwards, if the player doesn't have Invincible on and they are using a Garment Grid that has Break Damage Limit, the player should quickly use that gate. This is also worth doing for No MP Cost (the Last Resort Garment Grid has that as well) or use Three Stars to get the same effect.
Paine should use Nonpareil three or four times to increase her attack. Afterward, she must focus on using Cactling Gun, with Yuna using Moogle Beam. Rikku has two roles since she will have to deal with Numerus casting Protect and Shell on his heads. When both have been cast, the player should dispel all but one of the heads. Leaving one head with Protect and Shell on will mean Major Numerus won't cast it again. Paine and Yuna should concentrate on one head at a time. Rikku's second role is to chain her physical attack with Cactling Gun and Moogle Beam.
With this strategy, Major Numerus can be beaten in around 18 minutes.
A strategy that doesn't involve invincibility is to have all three girls equipped with two Iron Dukes on either the Higher Power or The End Garment Grid. The girls should start in the Thief dressphere (First Strike must be learned) and then activate the gates necessary for Break Damage Limit as soon as the fight begins; they should end up on a Mascot node. Yuna must immediately ready Moogle Cureja, Rikku must use a Stamina Tonic to double the party's HP, and Paine should use a Megalixir. All three girls should prepare to heal with Moogle Cureja or Megalixirs to survive the upcoming attacks.
With around 40,000 HP, the party should survive both Unum's Ladis Command and Quattour's Mortis Command. Yuna must continuously use Moogle Cureja while Rikku heals with Megalixirs and Paine readies for battle using Nonpareil and maybe Hayate. Paine should be the only one focusing on Numerus, as constant healing from Yuna and Rikku is essential. After using Nonpareil five times, Paine should use a Dispel Tonic on Unum to remove his Protect, then start using Cactling Gun. If Unum recasts Protect, Paine must dispel it again.
Unum has 555,555 HP, so 7–8 Cactling Guns should take him out. At this point, the party no longer has to fear the devastating Radis Command, and no longer has to throw Megalixirs constantly, as Moogle Cureja should suffice. Paine's next target should be Quattour, because he is the one using Mortis Command. She should dispel his Protect. With sufficient healing from Yuna, Quattour should fall in a reasonably short time.
At this point, Major Numerus's most devastating attacks have been eliminated. The player can now be more offensive, casting Moogle Regenja to put all three girls in Haste and have Yuna and Rikku be physical attackers, healing from time to time, while Paine continues to use Cactling Gun.
This strategy will require around 30 Megalixirs as well as significantly more time than the other strategy. Having two accessories that give +100% HP is mandatory. Two Iron Dukes per girl is ideal, but to make up for missing a second Iron Duke, it must be replaced by Key to Success or, failing that, Crystal Bangles (the latter, however, will be more difficult).
The captured fiends and/or humanoids that have incredible stats, like Almighty Shinra, are recommended, but they must also be equipped with the Mascot dressphere (as it makes them immune to status ailments), two Iron Duke accessories (which maxes out almost all stats), and the Hero's Stash ability (which grants the Hero Drink effect) to stand against Major Numerus's many attacks.
The player's captured fiends should be taught Fireworks, have Break Damage Limit and maxed Strength (255), and be at Level 99 to be able to deal massive damage to Major Numerus (around 43,000 with the Cheer Meter set to 5 stars). Like most enemies, Mushroom Cloud can trivialize even Major Numerus, as it can lower the boss's attack and magic power with Pernicious Powder. Couple this with a Machina Leader that only uses Impale and can deal over 42,000 damage with each hit, and Paine using Cactling Gun or Yuna using Moogle Beam (plus the extra Black Sky from the Mushroom Cloud if it has access to that ability), and Major Numerus will easily fall.
Using an L-sized fiend, on the other hand, is much more difficult, as equipping it with The End Garment Grid (grants the Hero's Stash) and teaching it Vigor, Auto-Life, and either Fireworks or Hurt is practically mandatory to beat Major Numerus. This is compounded by the fiend needing very high stats all around, at least 70,000 HP (preferably at least 85,000), and Spellspring to never run out of MP from using Hurt and Fireworks repeatedly. The Adamantortoise is a good choice as it has the highest base HP of all creatures (at Level 99, when equipped with two HP +100% accessories and the Mascot dressphere, it has well over 82,000 HP). The Adamantoise is the next best option; although the Jumbo Cactuar has higher HP, the Adamantoise has much more balanced stats, especially in Defense and Magic Defense.
Numerus is Latin for "number". Unum, duo, tria and quattour mean "one, two, three, four" in Latin respectively.