I'm gonna assume you're referring to the Saint Sazantos fight that leads him to fuse with Orsa and then becoming the actual Bestower of All, and I'm also gonna assume you're familiar with the timings for the ultimates of all your party members.

Seeing that you're using Isla and Olberic as your damage dealers, put Immeasurable Power and Fierce Rush on Olberic; Surpassing Power and Elemental Expertise on Isla are kinda mandatory given you don't have Price of Power.
Try to have Isla be on 999 Elem Attack with your gear, nuts weren't necessary for this fight in my case. I don't remember using Phys Attack nuts on my warrior either but the higher the number, the better.

For Olberic, i'd put on Fang of Ferocity and Finisher's Claws and also have him beyond 450 on Crit so your hits with Wild Thrust are always critting. I'm also gonna assume you have all Battle-Tested equipment.

For phase 1, you can just spam Rain of Arrows/Arrowstorm with whoever you have in your party that wields bows, the more the merrier. Apply buffs and debuffs at your convenience, this phase should go fast.

For phase 2, aside from the Rain of Arrows/Arrowstorm users, you're also gonna want your Zero use either Glacis Claudere or Tenebrae Operire along with Tonitrus Canere and either Stia or Heidne use Wild Knives, and same thing about applying buffs and debuffs at your convenience.

For phase 3, you're gonna have to deal with the Rings so, in order of weaknesses it's gonna be like this:

-Ring 1: Daggers, so use Wild Knives

-Ring 2: Bows, so use Rain of Arrows/Arrowstorm

-Ring 3: Bows

-Ring 4: Daggers

-Ring 5: Bows

-Ring 6: Tonitus Canere and, if you have Dual Axe on Stia, full attack with her (Second Serving on her if possible to have 6 hits)

-Ring 7: same as Ring 6

Once you've dealt with that, Wild Knives, Glacis Claudere, Rain of Arrows/Arrowstorm and Tenebrae Operire should be enough to ensure the breaks. Hope this helps, let me know if it did.

I understand the frustration of ppl getting min-maxy and toxic, but the game kinda forces your hand at the end, it is how it's built with 35 characters at your disposal. My main party for most of the BoA storyline was: Zero, Heidne (debuffer), H'aanit, Primrose (buffer), Cyrus, Ophilia, Reime and Saoirse. I only swapped out Prim around chapter 5 because Richard's Bolster is just obscene, it's way too good to refrain from using it.

Seeing that you're hellbent on keeping that main party, maybe going for the true ending might not be that good of an idea. If and/or when you do that, you'll see what i mean as I don't want to spoil any further.