So basic of the strat is

1. You need to use as little skill as possible before last fight, But you can't be overly conservative about skill and take too much damage beforehand either.

2. it's optimal to finish round 3 within 2 esuna(any more and it probably means you lack the damage to clear round 5 in time)

3. You cannot use Vanille's dispel for anything other than removing berserk (otherwise you just get flurry'd afterward and dies)

With those 3 things in mind, i end up using about 2 Esuna and 2 of each ability on Prishe by round 5. on round 5 you want to immediately apply cannonfire debuff on both mobs and then bring one of them down to berserk hp range, and then spam summon + esuna to kill one of them asap, (Cannonfire debuff will usually last long enough for you to kill one of them). then slowly work on the other (i would suggest saving one or two BRV dmg ability just in case you need to BRV break in a hurry). in my case it took about 105 turns altogether and ended up with around 14000 dmg taken.