In this fight, Yunalesca is accompanied with 2 Cursed Bones, and 2 Wind Elemental.
Yunalesca can do a single target attacks that can inflict poison, or an AoE attack that can inflict confusion. Having high faith can be dangerous if you don't have any confusion resistance. You can equip your units with Elfs Cloak to prevent it.
When you attack Yunalesca, she can deal counter attack damage and buffs herself with a physical and magic shield for 3 turns. Having units like Auron or EX Robb Hourne can help break the shield, or maybe any unit with Dispel Counter.
FFX related VCs and Dragon King Advent, Bahamut also get bonuses that stacks together. If you rolled for the FFX units, it's recommended to raise them up and use them for the fight.
Other good units to consider:
Ildyra can chain together with Tidus for water attacks.
Niv Lu to slow the boss and speed up your main damage dealer.
High Points and Auto
Having the bonus units and VCs are important for getting higher pts, but if you don't have them, the next best thing you can control is having high chains, and Tidus is a great unit for that if you managed to raise him.
Tidus has Delay Buster to slow the boss (but it has to crit), and can cast Hastega on himself. This lets him perform more actions before Yunalesca. At job level 12, he can use Quick Hit to keep attacking the boss to rack up the chains. The Limit Burst can be used at the start or as the combo ender for +3 attack and elemental chain. The rest of your teams can be either water units, supports to Steal Time, or anyone with Quicken.
If you want to auto the fight instead, it may not be consistent, but it's a lot more convenient and hassle free. Anyone with Green Mage can really help the most. Yuna might be the ideal one since she has Imperil, Deshell, Deprotect, and a revive (Full Life at EX). Otherwise, since the fight is fairly manageable without a tank, you can use a mono element team full of damage dealers so you can chain your attacks with 1 healer that can revive. The important thing is to have some confusion resistance on your important unit(s).
Yunalesca Stats & Resistances
Max Level Stats
Poison Up, Accuracy Up, Critical Evasion Down, Casting Time Reduced.