5 retaliate cheese all the way. Would have one shot her but she got off a good stock break at low enough HP I couldn't even see her bar. 2nd try crushed it like it was nothing. I may have gotten lucky but I really doubt it.
Auron: Double Cut / Retaliate / Light's Wrath (for holy req)
Tyro: Boost / Retaliate / Double Hit
Gilgamesh: Armor Strike / Retaliate / Auto-Haste
Sephiroth: Double Cut / Retaliate / Winning Spirit (critical: haste is more damage than auto-haste for this fight)
Cloud: Tempo Flurry / Retaliate / Untapped Power (tons of attack after first climhazzard)
RW: Luneth Advance
Turn 1 = 5x retaliate
Turn 2 = Auron attack for holy requirement, Tyro boost Cloud, Gilgamesh Armor Strike, Sephiroth double cut Cloud, Cloud Advance
Turn 3+ = Go nuts multi-swinging at Cloud until she dies (probably need to refresh retaliate on everyone around the 50% mark to be safe)
Just need to get a little lucky with her AE attacks at the end. Can still master with 1 dead so as long as she sticks to holy shock or missed stock breaks you'll be fine. As long as you have 1 FF9 synergy weapon she shouldn't get off more than 1 attack in that last phase. Easiest ultimate fight by far since they allowed the cheese option. Difficult (and probably much more fun) to do it the real way but I generally take the path of least resistance. For example, I advantaliate everything with this beast of a team:
80 Lenna: R5 Curaja / R3 Protectga / Double Hit / Shared SB Medica (or default group regen situationally)
80 Tyro: R5 Tempo Flurry / R5 Boost / Mako's Might / Sentinel's Grimoire
65 Sazh: R5 Double Cut / R3 Full Break / Dr. Mog's Teachings / Boon
65 Celes: R5 Double Cut / R5 Drain Strike (or mastery condition) / Ace Striker / Runic Blade
80 Cloud: R5 Tempo Flurry / R5 Retaliate / Dragoon's Determination / Default
RW: Advance
Turn 1 = Protectga, Wall, Shellga, Hastega, Runic, Advance
Turn 2 = Double Hit, Boost, Full Break, Mastery Condition, Retaliate
Turn 3+ = 10 swings from Cloud with boost/advance/armor break = 100k damage per turn
I'm fairly confident no other build can put out as much damage while retaining all this mitigation. The only thing that does more damage is SSB/BSSB meta but then you're forced to go relatively glass cannon on mitigation. Also, Celes' Runic is hilariously broken on about 30-40% of bosses. So I would have to go out of my way to force difficulty into the game and I'm ok with not doing that. The nostalgia is just as strong and that's what this game is really about. Hell, cheesing ultra-difficult bosses only brings back the nostalgia harder. FF6 Genji Glove / Offering, FF7 8x Knights of the Round, etc.