Trial Mode is an addition to the International Zodiac Job System (IZJS) and Zodiac Age versions of the game which offer you more difficult battles besides the end game content. The last Stages (specifically 95-100) are some of the most difficult battles in the game.
You can choose to partake in Trial Mode at any point in the game and any of the items that you steal or that you obtain from treasure chests will be carried over to the regular game. Thus, the Trial Mode can be used to level up your characters quickly to such a degree that it makes the regular story mode extremely easy. It is advised that you avoid Trial Mode and its rewards during your first playthrough of the game as it simply makes the game too easy.
You will be given the option to save your game after completing 10 Stages (IE at the end of Stage 20, 30, etc.)
Details on how to complete each of the Trial Modes is included in the guide sections below:
Stages 1 to 10
Dire Rats, Thextera, Flowering Cactoid, Razorfin, Nekhbet, Cluckatrice (and Chickatrices), Judges, Imdugud and Bagolys, the Cultsworn Lich and Liches, Belias
Stages 11 to 20
Croakadile, Melt and Jellys, Tiamat, Panthers, Elder Wyrm and Treants, Mateus and Ice Azers, Mindflayer, Ring Wyrm, Earth Tyrant Vultures
Stages 21 to 30
King Bomb and Bombs, Topstalk and Deadly Nightshades, Pumpkin Star and Pumpkin Heads, Madragora Prince and Madragoras, Onion Queen and Wild Onions, Alraune King and Alraunes, Wendice and Strikers, Ahriman and copies, Gil Gnapper and Storm Elementals
Stages 31 to 40
Bansat and Seeq Fishers, Zalera and Dead Bones, Scythe Mantis, Marilith and Bogeys, Adrammelech and Skulwyrms, Dreadguard and Mirrorknight, Behemoths, Mythril Golems, Shemhazai and Oses, Gavial and Baritine Crocs
Stages 41 to 50
Orthros and White Mousse, Cúchulainn and Foobars, Deathclaws, Antlion and Killer Mantis, Vishno and Brainpan/Deidar, Purobolos, Aeronites, Tower and Zombie Warlocks, Hashmal and Reavers, Famfrit and Abaddons
Stages 51 to 60
Helvinek and Oversouls, Catoblepas, War-chief Supinelu and Garif Adventurers, Deathscythe and Reapers, Tyrant and Piscodaemons, Larva Eater and Elvorets, Helm-Rooks/Spinner-Rooks/Sphere-Rooks, Trickster and Chocobos, Ba'Gamnan, Monid with Bangaa Hunters and Rabanastre Watchers
Stages 61 to 70
Baknamy, Vorpal Bunny, Ozmone Hare/Mu/Wyrdhare/Fury, Humbaba Mistant and Vorres, Deathgaze and Vyraals, Master Joe and Bangaa Pugilist, Shield Wyrm and Dheed, Exodus and Golems, Wild Malboro and Cassie, Carrot Vivians
Stages 71 to 80
Krjn and Viera Hunters, Diabolos/Avenger/Overlord, Slamand Entite and Fire Elementals, Mardu Entite and Storm Elementals, Leshach Entite and Ice Elemental, Gnoma Entite and Earth Elementals, Undin Entite and Water Elementals, Sylphi Entite and Air Elementals, Luxollid and Diakon Entites, Vagrant Soul and Leamonde Entites
Stages 81 to 90
Zeromus and Dark Lords, High Reavers and Reavers, Giruveganus, Chaos/Air Chaosjet/Earth Chaosjet/Fire Chaosjet/Water Chaosjet, Pylraster/Abelisk/Terror Tyrant, Rikken/Elza and Pirates, Hell Wyrm, Disma and Forbiddens, Fafnir, Behemoth King
Stages 91 to 97
Magick Pot and Hecteyes, Shadowseer/Fenrir/Slyt/Pandaemonium/Phoenix, Lv.99 Red Chocobos, Gilgamesh and Enkidu, Ultima, Abysteel and Glaring Eye, Zodiark
Battling and defeating Yiazmat.
Battling and defeating Omega Mark XII.
Defeating Judge Gabranth, Judge Ghis, Judge Bergan, Judge Drace and Judge Zargabaath.