Sorry my mistake, not the Tiamat but the Elder Wyrm. It's just after going to the Henne Mines. He looks like a dragon covered in grass and to treants attack you too. He does a Fireball attack which does alot if you have the oil status. His worse attack is Sporefall which inflicts a variety of status change on you.
Right now all my characters are 21 and leveling up again before fighting him.
Oh yeah, I remember that bastard. Ok, you want to have your healer person equipped with something that has status: immune to silence, because when he drops all those status's on you, at least your healer should be able to cast esuna. In fact, make that your first gambit for the healer. It should be:
ally:any--------esuna
Also, the Sporefall attack will only affect your characters if they are next to the boss, so when you see the sporefall coming, move your character to the edge of the battle ground or just as far away as possible from the boss and you should not be affected by it.
The way I had it was that my healer person had a bow and arrow weapon, so she could attack from a distance. Therefore, when he dropped the Sporefall I moved the character I was controlling away from the boss and only two of my characters were inflicted with all the status effects.
