Defeat shadowseer

When he uses invert you could try using reverse so if he or one of his monsters tries to hit you will gain health, instead of dying. Or have someone ready to use curaga/curaja. You can put some of his pets asleep, I think it's fenrir, and that fish. While their sleeping the seer won't attack you. So you can use all the spells you need, if you have the mp.

You should also be above 60 before you fight him, I was on 80 and it was kinda hard.

Oh, you can also use x potions for really quick healing. Or when he uses invert on someone switch to that person, and run back so he can't target you, and switch, or just remove him from the party. That way you can heal him when he's in the reserves.

Equip armor that lets you absorb his elemental attacks, and converts it to HP. That would keep your HP up, even when he does use invert. Or just use a ether to make your MP as high as possible before he does it, so you will still have a chance of surviving afterwards. Decoy might also help.

Hope this helps.

I like to equip white masks, and Sage rings on my characters. They don't need to neccesarily have all, and then I give one person Excalibur and equip them with a Cat-ear-hood, which really jumps up their speed.

The part should be equipped with a Ally:Any-Vaccine gambit, since disease is one of the main problems, and this should be ranked even possibly above a phoenix down/Arise gambits.

The character with the Excalibur should also have an Ally:>X% HP-Attack gambit. With all members equipped with sage rings and white masks it will be possible to quickly heal party members by actually having this character to quickly deal holy damage to your allies and itself, which will allow you to quickly heal part members without MP, or other items which item use should be limited to Phoenix down use on the immediate party and reserve members, as well as Vaccine use.

This worked for me and Level 70 for my main party, and 41 for my reserve members which were underleveled.

He's impossible to defeat if you don't have the epser ultima and the weapon zodiac spear, if you don't have that.. Then just don't try to fight him.. Because you will never win