Lv44-48 for others, 54 for Zidane.

Don't use physical attacks on Tiamat, he will blow you away. Keep using skills with massive damage(magic is preferred) and he will be down in no time.

To let you prepare for the later ones, don't equip return magic when fighting Kraken. He casts Waterga often and it heals him.

As for Lich, just keep up your HP.