The Judge Magisters are 5 very difficult Trial Mode superbosses.
Stats
Gabranth - 2378-2417 attack
Zargabaath - 1503-1542 attack
Bergan - 2503-2542
Ghis - 2003-2042
Drace - 2128-2167
How to Summon
Kill Gilgamesh 2. They will spawn in a neat formation and say "Trial Mode 100" to you. Even though they say Trial Mode, there is no Trial Mode that must be activated in order to fight them. Just play regularly.
*( 0.9.4 Patch, You dont actually need to kill gilgamesh2 first, You just need to unlock dimensional cup, and then beat it for 4 times, it usually drops Judge's Helmet or something like that. Bring the Judge's Helmet to the place where you put Perfect Mark of Darkness for Penance )
Spells
Gabranth - Speed Break, Armor Break, Magic Break, Charge, Lancet, Power Break, Solid Skin, Disable ("Prevents the chosen enemy doing anything")
Zargabaath - Wave Fist, Bolt, Holy, Flare Star, Firaja, Solid Skin
Bergan - Holy Wrath, Improved Critical Strike (15%, 3x), Solid Skin, Wave Fist, Rage, Iainuki, Omega Skill (crit+evasion), Holy Power.
Ghis - Blind, Slow, Stop, Poisoned Dagger, Solid Skin, Disable
Drace - Haste, Inner Strength, Praise, Charge, Regen, Shell, Protect, Solid Skin.
Battle Strategy
If you do not know how to approach the fight, this can be the hardest in the game.. There are five of them, and while their damages are modest on their own, together their damage totals 10515, more than Strongest Shinra. Also, they have a huge variety of spells, including 4 stuns. In addition, they each have a Pocket of Elixirs that can heal any of them to full HP with no cooldown and they also each have a single Megalixer, capable of healing all of them to full HP and MP.
However, their numbers can be used against them too, making this a very easy fight. On Normal, Ghis and Drace will only have 60,000 HP which allows you to destroy them in one hit with Odin Materia, and perhaps also Tonberry Materia depending on your kill count.
You can defeat all of them in one shot with odin+tonberry. You will need enough mana to use Odin, one Hero Drink (Or Hidden Hero Medicine), 4000 kills, and one Elixir. First, use your Hero Drink, run up, and use Tonberry on Ghis. This should kill Ghis and make him explode, lowering all of them. Then, use Odin which should kill the rest of them. If you do it correctly, they can't Megalixer, but if you do it wrong, they will heal. Check the video, it was done in 46 seconds there.
Drops
Upon victory the judges will exorcise the Judge's Helm, yielding the Helm of Divine Judgement (Gives 5000 HP, +25 HP/sec regeneration, 75 Strength, 50 Agility and gives a 30% chance to block physical attacks).