Anustart:
I mentioned to you some posts back: You want fire resist accessories and wind resist magicite because Savage Jet Fire, which is Fire/Wind and is non-piercing, is her most dangerous attack.
Mr. Daravon:
If you think your ice output isn't sufficient, go back and kill Isgebind until you have a L99 Isgebind and a L99 Dullahan. You'll also, ideally, want Wind resist magicite (Krysta) and Fire resist magicite (Gizamaluk), though the 3* versions will suffice.
Run the following:
Squall: Life Siphon, N/A, Gathering Storm (BSSB2)
Celes: R1 SSS, R3 SSS, Love Wake (USB)
OK: Chain Blizzaga, Entrust, Mako Might (BSSB)
Elarra: Protectga, Curada, Ace Striker (USB)
Rinoa: Voltech or Chain Blizzaga, Valigarmanda, DMT (CSB/AOSB)
Main Magicite: Wendigo
Do you have OK LMR? It will make things a bit simpler since it will allow him to use BSSB and RW Fabula Guardian in two turns, before Tiamat's first Twister. If not, Squall will have to cast RW turn 1 before Life Siphoning.
Otherwise, here's the rough battle plan:
Squall: Life Siphon x3, BSSB2, CMD1x2, CMD2xN, finish with USB2.
Celes: SSS xN, USB, SSS xN, USB finisher
OK: BSSB, RW, Chain Blizzaga, entrust Elarra, Chain Blizzaga x2, Entrust Rinoa, Chain Blizzaga xN
Elarra; Shellga, Curada, USB, RW, Curada xN, USB when available
Rinoa: CSB, Valigarmanda spam, refresh CSB, Vali/Chain Blizzaga (or Voltech if you have it) spam, AOSB
Goal is to inflict as much damage as possible from the start to push phases. Key will be managing Rinoa's SB gauge since she has your biggest finisher. If you can win in one chain it would be better (in which case, you do CSB, Valigarmanda/Chain Blizzaga or Voltech spam, then AOSB before chain runs out) but I'm not certain if you're quite strong enough. When in doubt, look to refresh chain and have OK entrust some half bars to Rinoa to give the extra push.
If Squall gets hit with Power Absorb, Restart.
IF you find that you can't survive with Elarra, use Rosa with Wrath/Protectga and Ace Striker. She'll be doing Wrath x2, USB ad infinitum. It won't protect from the (Savage) Jet Fires but you'll get free turns with Savage Twisters.