Its a Fire based aoe attack, fairly certain +/- pdt wouldnt matter, depends what job ur tanking it on but minerva's is a useful ring. Ideally it would be defending ring and shadow ring, allowing you to cap mdt whilst still maximizing haste and mdb but pre dring a 5-6 mdt dark ring and 8 mdt minerva's will contribute to a decent chunk of your required mdt. As for a mob as old as tiamat any job in a mdt set and barfire could take minimal damage when its airborne, buying fire resist gear for this definitely is not needed. Having said that there is one piece id use suzy feet augmented with + mdb.
Feet +2haste +2mdb
This is what I used this weekend just gone when tanking an airborne Wyrm Ks99.
wiki :
The Wyrm will take flight when it goes below 66% HP and will stay up until it gets down below 33% HP. While in the air, it will not use Dragon Breath, Absolute Terror, Horrid Roar or Spike Flail, but it gains the use of
"Flame Blast - 35' AoE Fire damage (~900 before reductions).
While in the air, its normal attacks will look like a fireball coming from its mouth, and will do Fire damage (~325 per hit before reductions) instead of physical damage.
~220 total Fire resistance (from gear, Carol and Barspell) is enough to usually reduce its normal attack to ~12 damage, and also dramatically reduce Flame Blast damage.
329 total Fire resistance from gear, Carol, Barfira, and Shell5 was able to reduce its normal attack in the air to 0 damage. It was also able to nearly nullify its flame blast damage
Magic, Ranged Attacks, Chi Blast, Jump, High Jump, and Avatars can damage it quite well while it is in the air. Melee attacks will not work. "
And still took next to 0 dmg.