This one for me by far
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and I tried out something new in ch 7 and 8 since VERY MINOR SPOILERS (none to the story at all; just about the fights in general)
Boss fights have you at 3-4 enemies at a time)
Mnk/Vamp
-Natty Talent
-P. Dmg + 10%
-Speed + 20% (Needed for my particular group for turn order)
-Monster Ability Up
I spend the first round absorbing attack/magic attack from enemies to reduce damage taken (especially since not wearing any armor)
After that; highest overall damage I could personally come up with.
Firestorm x4 on 4 enemies = 159,984/round; Once every 4-5 rounds, you'll need to replace one Firestorm with a magic thirst though.
Then I keep a Ninja for single target phys dmg to focus on taking a particular enemy out first (7,000-9,999 depending on buffs x2 per round for 3 rounds (Ikik takes up a round) = 42,000-59,994 dmg/turn on a single target
9999 x6 isn't that much more dmg over 9999 x4 when it comes to a single target really, but nin is also useful for utility when things get hairy, I'll use it to make sure my tank is the target for those rounds, and I have support abilities to compounding, so I can aid in items (whether it be Giant Drafts for double HP, resurrections, or general healing items)
My general strat has me at 3 BP/turn on every char, so the Ikik attack x3 and Firestorm x4 are generally an every turn thing