Octopath Traveler COTC [SEA] Thief Training EX5 (5F EX1) (First Clear)

To be honest, I'm not sure what I did in this fight. Various RNG and a lot of mistakes happen. But first clear is first clear. Should be happy about it.

Saying "extremely annoy battle" is more accurate than "say it hard."
This tower's gimmick is about debuffing. The boss just enjoys spamming a lot of debuff like paralyze weakness and silence. And she can act 2 times per turn.
She also increases shield by 3 every time she recovers from breaks.
When her HP cut to the yellow zone. Sometimes she locks 3 of her weaknesses. and sometimes "dagger" is that locked weakness.

The boss will act 3times per turn once adds is gone.

Even worse, ADDS loves to increase boss shield by 3, and buff Steadfast Defense, which keeps you from cutting more than 1 shield in a single act.
I don't know of any mechanism or pattern behind that. However, if you have confidence of the character's toughness. Taking off the adds first could be one way to go.

Due to a number of bothersome factors, the game has just given birth the catastrophe class of RNG battle.

Boss weakness (Shield 25 +3every time she breaks)
dagger, axe, fire, wind, dark
Adds weakness (Shield 15)
sword, dagger, ice, lightning, light

Since there will be a lot of RNG. I am unable find a simpler approach to clear this floor than handle the current circumstances.
However, I may recommend trying to break the boss the turn following the adds break. It helps you in avoiding a scenario in which the ADDS raises the boss shield from 1 to 4 and ruins your strategy to break.

The majority of the boss attacks deal Physical damage. so equipped for the P.def focus equipment. And ensure to equip HP Regen accessory.

For character we have a lot of debuffer
Atk debuffer
Viola, Auguste, Falco, Therion, Kurtz
Def debuffer
Viola, Nicola, Auguste, Falco, Therion, A2, Cless, Billy

DPS
Heathcoat, Auguste, Falco, Therion, A2, Ri'tu, Adelle

Damage boost backpacker
Nicola, Falco, Wingate

Healer
Kurtz