If you're like me and don't bother with minmaxing every turn and just hit Repeat every time, maybe you won't one-shot the last stage and get a nasty dual Ultima to your bonus units' faces.

Equip Manufactured Nethicite if you have some shit unit like Alma and use it before hitting Repeat again on the last battle. Those Ultimas won't bother you again.

EDIT: I've personally tried it myself. It blocks just one Ultima, enough to survive and hit that Repeat button again. Especially useful if you're using Rikku's Pouch or World Destroyer, as her Ultima's damage doubles if hit with a break

EDIT2: See Solaris's comment for the AI. Apparently she only casts Grand Cross every 4 turns or if hit with any debuff (imperils, breaks). Her Ultimas also get scarier.

If she doesn't have any debuff, she will only perform 2 actions per turn, any combination of Ultima or normal attack. So yeah, don't debuff her if you cannot one-shot.