Can't get past second boss in The Sunken Temple of Qarn (Hard)

How are you suppose to beat Sabotendar Emperatriz? I'm playing as a level 51 Black Mage, and I have full level 110 equipment, all obtained from the Wolf's Den. My party always loses to this guy. He summons his minions, they overwhelm me, he reaches the 75,000 Needle attack, everyone dies. I have a feeling I'm never going to get past this, and this is part of the main quest after beating Ultima Weapon. Maybe I should try another class in the party, but I can't count on it. What equipment should you recommend for this as a Black Mage? What about the type of strategy I should do as a Black Mage? Type of materia?

Kill the tethered one and when the boss is counting needles hit her to interrupt it.

It's only a matter of knowing the order to defeat the enemies. Focus damage on the boss to interrupt its needle attack. Part way into the fight, one of the adds uses a Cover move on the boss (tether together) to protect it from damage, so you defeat that specific add asap then resume hitting the boss to again interrupt the needle attack. All other enemies are optional to defeat.

possibly limit break Guardia when he appears.
Before that kill compeador to take some strain off, he is a pain.
And as soon as she starts the 25 000 needle counting you hit Imperatriz to interupt her.
Do some aoe during all that to kill some little ones to make seeing and targetting a bit easier.

The only two mobs that matter in this fight are the Imperatriz and the Guardia. Dealing a certain amount of damage to the Imperatriz during her super cast will interrupt it. The Guardia will cover the Imperatriz. The rest of the mobs don't do that much damage. A healer can tank them while everyone focuses on the boss.