Kill the fat one in the middle first. It will be difficult, but you have to do it. They can't use Delta Attack after you kill her, so it will be worth the effort. In general, have Cecil, Cid, and Yang fight. Have Tellah Cure any party members that don't have a wall on them.
ATTACKS: Regular Attacking, Delta Attack(a different high level spell each time it is used), Fire 1, Cure 2(on themselves), Wall(on themselves, but it will effect you too)
If Rosa knows it, have her cast Bersk on at least Cecil. If not always have him fight. Also have Cid and Yang fight. Always have Kain jump, even if Valvalis isn't spinning. If she is, Kain jumping on her will stop her spin. Have Rosa Cure and Heal people.
ATTACKS: Regular Attacking, Ray, Weak
I will not even post a strategy for this fight. No matter what you do, he will explode, and at least one of your party members will be dead. If you take out Lugae and Balnab at the same time you don't even have to fight this.
ATTACKS: Regular Attacking, Explode
You MUST try to take them out at the same time. If you don't, Balnab(Z) will eventually explode, killing one of your party members. Have Rydia summon Mist. Rosa should heal. Kain should jump on Lugae. Cecil should attack Balnab. Yang should use his kick ability. If you do this, you should be able to kill them both at around the same time.
ATTACKS: Lugae will Heal Balnab, Balnab will use regular attacks, and if you take out Lugae first, Balnab will explode.
To quote the good doctor, now the real fight. He's much tougher here because he changes forms. He'll poison you and put you to sleep, so be careful. Cecil shoud fight. Rosa (as always) should be healing. Have Kain jump, and Yang fight. Rydia should be casting some strong spells.
ATTACKS: Regular Attacking, Poison, something that puts you to sleep, and he'll heal you.