Here is my Dark Nebula/Stillness loop. Three classes will do it: Ninja/Spiritmaster , Ninja/Dark Knight, Dark Knight/Spiritmaster, Spiritmaster/Time Mage
Essential (but not necessary) equipment: 2 Hermes Sandals and 1 Hermes Shoes and 2 Dark Shields.
This setup revolves around the Dark Nebula, Stillness, and Hasten World abilities. I beat almost every boss in the final chapter "Unscathed" (never took damage).
1st character: Ninja with Spiritmaster
EQUIP: Hermes Shoes and Dark Shield
ABILITIES: Speed +10%, +20%, Equip Shield. (Equip a Dark Shield to become immune to Dark magic, or cast Adaption on self every round to absorb Dark)
BATTLE: This character should/will always act first in battle. Have them cast Adaptation (Absorb all elements) to the 2nd and 3rd characters and / Fairy Ward (Immune to all statuses for all). This character will never attack. This character's only role is to keep up Fairy Ward and Adaptation to the 2nd and 3rd characters. If you don't have the Dark Shields then just cast Adaptation on every character who doesn't to keep them alive during the Dark Nebulas.
2nd character: Ninja with Dark Knight
EQUIP: Hermes Sandals.
ABILITIES: Speed 10%, 20%, Two Handed, Gloom
BATTLE: This character must have high speed but have less than the first character/ninja. You want them to act second in every battle, because the first character needs to cast Adaptation on them to absorb Dark. This character will Brave four times and cast Adaption every time. It should hit 9999 damage to all enemies and allies, healing yourself every time so you don't lose HP when using Dark Nebula.
3rd character: Dark Knight with Spiritmaster
EQUIP: Hermes Sandals
ABILITIES: Speed 10%, 20%, Two Handed, Gloom
BATTLE: Depending on the speed of your 4th character and the enemies you are fighting this person's battle strategy may change. Because the two first characters are Ninjas they should be faster than this Dark Knight, but your Dark Knight should still be very fast (at least faster than every enemy). He has two options depending on the speed of the enemies:
1: Brave x3 and cast Dark Nebula, Dark Nebula, then Stillness.
OR
Brave x4 and cast Dark Nebula
4th character: Spiritmaster with White or Time Magic
EQUIP: Dark Shield
ABILITIES: Hasten World (Time Magic Ability) and Speed 10%
BATTLE: Depending on this character's speed they should act fourth in every fight. Sometimes if a boss has good agility they will act fifth, meaning the battle will go; 1st Ninja, 2nd Ninja, 3rd Dark Knight, 4th BOSS/ENEMY, 5th Spiritmaster.
If the boss is acting faster than your Spiritmaster then you need to be sure that the Dark Knight character is the one casting Stillness. If your Spiritmaster is faster than all the enemies then they should be casting Stillness. Have them support with Time or White magic or restore MP to the first Ninja to keep up their MP for casting Adaption and/or Fairy Ward.
... During Stillness the bosses/enemies will pound on you but never be effective. As long as your first character casts the Fairy Ward you'll never get with status effects. Just Brave during the turns on Stillness and repeat the loop. You'll always be able to do the Dark Damage with Dark Nebula which will heal the characters using it, and Stillness will keep every Unscathed.