A Muscle Belt can bring the wearer's HP high enough to survive one Meteor if they couldn't already, and a Paladin's Shield or anything that can absorb Fire will help, as Firaga may then raise their HP to survive the next Meteor.
The Red Cap is another option to raise the wearer's HP, but note that a combination of max HP raising equipment won't have a better effect than the piece of equipment that gives the best HP boost would on its own.
Mog is a good character to use, with high Defense being potentially a better means of defense than high Magic Evasion, since Firaga is better absorbed than evaded.
Another tactic would be to go all out on the offense, since nothing can be done to definitively prevent Meteor. In this case, a Genji Glove with the two most powerful weapons at hand and a Reflect Ring would allow the player to deal good damage by attacking AND reflecting Firaga back at Great Behemoth.
Shadow in the front row equipped with the Kagenui and Ichigeki from the Genji Glove and Reflect Ring can do good damage, with a chance of casting Stop on Great Behemoth, reflecting Firaga, and countering Great Behemoth's physical attacks with Interceptor.
[view·edit·purge]Behemoth is a beast mentioned in Job 40:15–24. In addition to mythological creatures, it is likened to elephants, hippos, rhinos, and bison. Metaphorically, the term behemoth denotes "an extremely large or powerful entity."