This is a cheap dirty method which in Easy and Normal should kill off the entire team in one turn (on Hard you may need Bravery Seconds, however this have not being tested).
You need your entire party as Dark Night Ninja - these are Ninja who have the Dark Art's ability as their second ability. You need to level up the Dark Night to learn Gloom and Dark Nebula (Dark Night Level 11). Switching them to this combination will do 2 things:
Ninjas give the biggest innate speed boost, allowing the party to get their attacks in quicker
Ninjas have innate Duel Wield skill
Equip every character with the 2 best weapons since this trick will end the battle in one turn. When the battle starts Brave x4 and use Dark Nebula. My party (at Level 79) did about 5,000 - 9,999 damage each attack, depending on what other abilities they had, their weapons and critical hits. After the party is finished they should have done between 80,000 - 159,984 damage to each opponent which should end the battle, so the fact that everyone is now on < 20% HP should not be a problem.
If this is too risky there is an alternative which has worked in future battles, however there is an margin of error in it. You have 3 Dark Knight Ninja's with the same set up as above and a White Mage Spiritmaster (Spiritmaster with White Magic Ability) who has the Holy One Ability (Spiritmaster Level 9), because the Ninja will act first your White Mage Spiritmaster can then use Curada, Holy One will boost it's power by x2.5 which will, most of the time, fully recover the party.
The margin of error comes from when the White Mage Spiritmaster acts, ideally you want them to act after your Dark Knight Ninja to recover the 80% HP they spent during their onslaught, but before the enemy has a chance to pick them off. The advantage to this method is that if you survive the end of the turn (and until you have 0 Braves) you can repeat the Dark Nebula x4 again later on.
(Kudos to Liz on Miiverse for coming up with the basic approach)