FINAL FANTASY X/X-2 HD Remaster

To unlock the Nemesis fight, you will need to capture 10 of each fiend in the game, and then defeat each fiend in the Area Conquest, Species Conquest, and Original sections of the monster arena at least once. To capture fiends, you need to buy weapons with the "capture" trait from the Monster Arena owner.

Fiends to be captured:

Besaid
Dingo
Condor
Water Flan

Kilika
Dinonix
Killer Bee
Yellow Element
Ragora

Mi'ihen Highroad
Mi'ihen Fang
Ipiria
Floating Eye
White Element
Raldo
Bomb
Vouivre
Dual Horn

Mushroom Rock Road
Raptor
Gandarewa
Thunder Flan
Red Element
Lamashtu
Funguar
Garuda

Djose Road
Garm
Simurgh
Bite Bug
Snow Flan
Bunyip
Basilisk
Ochu

Thunder Plains
Melusine
Aerouge
Buer
Gold Element
Kusariqqu
Larva
Iron Giant
Qactuar

Macalania
Snow Wolf
Iguion
Wasp
Evil Eye
Ice Flan
Blue Element
Murussu
Mafdet
Xiphos
Chimera

Bikanel
Sand Wolf
Alcyone
Mushussu
Zu
Sand Worm
Cactuar

Calm Lands
Skoll
Nebiros
Flam Flan
Shred
Anacondaur
Ogre
Coeurl
Chimera Brain
Marlboro

Stolen Fayth Cavern
Yowie
Imp
Ghost
Tonberry
Dark Element
Nidhogg
Thorn
Valaha
Epaaj

Mount Gagazet
Bandersnatch
Ahriman
Dark Flan
Grenade
Behemoth
Grat
Grendel
Bashura
Mandragora
Splasher
Achelus
Maelspike

Splashers, achelus, and maelspikes are found in the underwater section. Ahrimans, grendels, behemoths, dark flans, and mandragoras are found in the caves.

Inside Sin: Note that the point of no return is once you go inside the tower that comes crashing down, right after a save point.
Exoray
Wraith
Gemini
Demonolith
Great Marlboro
Barbatos
Adamantoise
Behemoth King

Omega Dungeon: Omega Dungeon is found by putting in the coordinates X=74.101, Y=36.099 into the airship.
Zaurus
Floating Death
Black Element
Halma
Puroboros
Spirit
Machea
Master Coeurl
Master Tonberry
Varuna

Many of the above fiends can be found in more than one area, but are classified by these areas according to the Monster Arena.

Once you have captured 10 of each, you need to beat the following fiends in the Monster Arena:

Area Conquest
Stratoavis
Marlboro Menace
Kottos
Coeurlregina
Jormungand
Cactuar King
Espada
Abyss King
Chimerageist
Don Tonberry
Catoblepas
Abaddon
Vorban

Species Conquest
Fenrir
Ornitholestes
Pteryx
Hornet
Vidatu
One-Eye
Jumbo Flan
Nega Elemental
Tanket
Fafnir
Sleep Sprout
Bomb King
Juggernaut
Ironclad

Original
Earth Eater
Greater Sphere
Catatstrophe
Thu'ban
Neslug
Ultima Buster
Shinryu

These monsters are some of the toughest enemies in the game. If you're underleveled, you should grind using the Don Tonberry trick. You will level a decent amount just capturing fiends, too. I recommend using zanmato to beat the harder bosses in the Original section if you're still having trouble. This can work for everything except for Shinryu, which is an underwater fight where you can't use Yuna. I had a max-damage Wakka use his Attack Reels overdrive with a break damage limit weapon, twice. At max damage, a perfect Attack Reels will deal 1.2 million damage. Use "Entrust" to fill his overdrive a second time. One perfect use of a damage-break Attack Reels can also one-shot many of the other fiends.

Once you have beaten all of the fiends, Nemesis will unlock in the second page of the original creations section. Using zanmato is the easiest way to beat him.

If you want to fight him normally, he has 10 million HP. He counters every attack with Ultima. All characters should have armor with ribbon equipped, since Nemesis' Ultra Spark casts Poison, Curse, Slow, and Power Break on the party. Auto-Haste armor should also be equipped. Nemesis' ultimate attack Armageddon deals 99,000 damage to the entire party, so at least one person should have auto-life cast on them. Use aeons for meat shields, and Quick Hit him until he goes down. Note that Kimahri can learn Nova from Nemesis.

Comment with any questions about other fiends!