I think it's better to fight Tantarian in Chapter 3 when you have more available party members. I would use Zidane, Freya, Quina, and Amarant, just because of their skills. But use four characters who can cause a lot of damage by other means than physical attacks. Vivi, Garnet and Eiko are also good for this, but they can require a lot of MP. It's your call, just don't use Steiner.

Have everyone equipped with Antibody, this is a must. Auto-Regen/Potion would be good since you probably won't have a healer in the party, and Freya's Reis's Wind can only do so much if you use her. If you are going to use Amarant, equip him with Poison Knuckles and Spare Change.

Ok, so Tantarian will start out as a closed book. You want to cause close to but not more than 200 damage. Amarant's Sparge Change has a good chance of doing this in one blow. Freya's Jump works well here too. If you go over 200 damage the damage total will reset. It has some pretty powerful moves in this form, I use Quina's White Wind a lot here. A summoner's cure/a/aga (I don't know how powerful it can be at this point) would be nice here too, but not necessary with Auto-Regen. To make this easier, make one character attack. Wait for that character to finish the attack to see if the book opens. If not, attack again. You don't want the book to open and do the next physical attack in line. This is because when the book opens, it's defenses go way down, but if you physically attack it the book will close again, and you have to start all over. So, when the book open, unleash the skills and black magic and summoning. Have the summoners summon. Have Zidane either keep Stealing or use Thievery, if he goes into Trance then use Dyne to speed the battle along. Have Freya use Lancet. Have Quina perform some Blue Magic, Frog Drop works well if you have powered it up. Have Amarant use Spare Change, it will do a lot more damage now. Have Vivi cast powerful spells. I believe Bio would be your most powerful spell at this point, I'm not sure. And the best part, in the open book form, Tantarian only uses a Bio spell on the whole party. But with Antibody, it won't hurt you at all. So kill it as fast as you can. If the book closes again by itself, you have to start all over. Don't bother stealing the Demon's Mail, the odds of success are not on your side at all, even with Bandit. You don't need one until later, and at that time you can simply buy one.

And that's it.