Since Belgemine is using Shiva, you cannot use her, but you have Bahamut to help handle things. Despite being an ice-elemental Aeon, Shiva here is not weak to fire, but she does absorb ice. She also has some pretty high defense and magic defense, so your attacks are not going to deal too much damage. However, Overdrives are going to inflict a lot more damage than normal, and Bahamut’s Mega Flare can even break the HP damage limit of 9,999. Shiva will use her normal attack, as well as Blizzara. If her Overdrive meter gets filled up, instead of using Diamond Dust, she will actually use Heavenly Strike.