The Four Seals are a very tough boss. I think I had someone cast Float on everybody and Haste2 on my White Mage, who then quickly cast Shell on everyone. After that, the White Mage will be busy healing. Make someone a Summoner and have him/her cast Golem and then use repeatedly Titan. This should kill everybody except the Earth Seal. Finally, have your physical attackers defeat the Earth Seal. Good luck, this is a really hard battle.