The Seymour boss rush is fucking ridiculous. You need far too many abilities.
You have to have:
death/pressure point/lai strike
all of the breakdowns, including power for the final boss despite the first 2 bosses using only magic
healing
diaga
Carbuncle
venom buster
silence buster
That takes up 10 of your 10 available abilities, and then you still need enough DPS to whittle them down.
I reached the final form but I have no abilities left, Carbuncle is completely useless, as is Death. the boss delivers 3 hits of around 1000 damage to random party members and I simply can't heal everyone fast enough, nor do I have the mitigation to lower the damage due to needing the slots for other abilities.
I got him down to half his health before he killed Tidus, Rinoa and Auron with a cross cleave followed by 3 physical attacks (one of which was a critical)
They aren't even hard bosses but you are pretty much forced into using the listed abilities. I'm not even sure if I can be bothered trying again, it's a fucking stupid design. Especially when it goes from 2 magic focused bosses to a dangerous physical based one. eurgh.
I was fortunate enough to end up with both a 5* and a 4+* FFX Sword, so I went with Advantaliate. Auron had the strongest sword, Tidus the second-strongest. I think my setup was something like this;
RW: Advance
Auron: Retaliate R5, Iai Strike R4 (RM: Sword+)
Tidus: Venom Buster R4, Tempo Flurry or Magic Break R4, I forget (RM: Holy on Attack)
Sahz: Power Breakdown R3, Silenga R3 (RM: Haste)
Yuna: Kirin R4, Carbuncle R2 (RM: Double Hit)
Yshtola: Diaga R3, Armor Breakdown R3
I used Advantaliate on bosses 1 & 3.
Boss 1: Make sure Iai Strike takes out one of the guard, and melt the other one with Diaga and physical attacks. Get Carbuncle up during first phase. After killing one guard with Auron, I popped Retaliate into Advance. All the various people in this dungeon have so little health that Advantaliate managed to melt them all before either Carbuncle or Advance ran out.
Boss 2: Diaga + Tidus's Holy damage, combined with some bonus damage from Auron spamming Aiai Strike, melted the boss. Throw up Carbuncle at the start.
Boss 3: Apply status effects, Power Breakdown both halves. Melt with Advantaliate.
While I did have both Sahz's Boon and Yshtola's Stoneskin, I don't think they played any noticeable part in my victory. Boon didn't even get charged until the third battle, and I don't think I needed Stoneskin either.