I'll recommend the same strategy for defeating Ultimecia as I do for just about every difficult boss around. Have one character summoun Doomtrain (if you have time, have another summoun Cerberus as well). Doomtrain will cast Meltdown on Ultimecia allowing your hits to do max damage. Have one character cast Aura on Squall (or everyone if you've also summouned Cerberus) and then use a Holy War. Keep hitting her with physical attacks, recasting Aura when Holy War runs out and then using another Holy War.
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