I have a strategy. First off, have a great third charachter. Second, make sure all three have at least
one good Hi speed ether. Also, make sure Shion and Jr. have boost 1. Okay, when the battle starts
have Shion&Jr. always use boost 1 on the third charachter. But... If you have low health have Shion use medica all. On the third charachter,s turn either restore ether points or use a tech attack. When Tiamat is next after Shion or Jr. boost the third charachter. When Tiamat has low health, *"Kill"* it using Shion/Jr. After it heals do the same thing. Kill it with the other Character

Here's a strategy for this battle:

The characters will not likely be able to withstand the attack-all and counterattack assaults of Tiamat on their own, so use each character's first turn to transmit them into their A.G.W.S. armors. Having equipped your mech suits with the proper anti-status abnormality accessories before the battle, building AP and launching W-ACT attacks should be no problem. When inflicted with status abnormalities, A.G.W.S. equipped with Guard Cleaners can simply defend for a turn to recover. Guard to recover FHP and remove A.G.W.S. status impairments with the use of accessories, and launch W-ACT attacks whenever possible. Through this method, it is simple enough to weaken and defeat Tiamat. However, there is a trick to finishing off this formidable monster.

Shion and Jr. should guard to build AP. If possible, use Boost1 Ether on both characters to increase their frequency of turns over the boss. Try to delay Tiamat's reaction to Shion's turn as long as possible or it will inflict massive damage on all the party at once. When ready, launch a triple attack ending with an appropriate Tech Attack. Determining the right attack to use is also tricky. The weakness of Tiamat changes depending upon which side is facing the party. The side featuring the tail arm is weak versus piercing attacks such as Prelude to Battle or Shock Blade. The other faces are weak versus Beam attacks such as Moonlit Serenade or Spell Ray. Shion or Jr. must be the character to perform the finishing blow to Tiamat, and neither can accomplish this from within their A.G.W.S. When such a message appears onscreen, Tiamat will recover 1500 HP and keep attacking. Remove your characters from their A.G.W.S. and start attacking conservatively. Keep the party alive by casting Helaing Dew every turn, or Medica All.

Once Shion or Jr. delivers a finishing blow, Tiamat helas itself and the other person must provide a finishing blow. Keep very careful track of how much damage each attack delivers, and make sure that the appropriate character is the one to finish off the monster.

Good luck!

-DragoonMaster