If you've learned the appropriate black mage skills, change your characters to summoners to increase their magic power, and use !Black LV3,4,5 whatever to cast.
This is what I said in my walkthrough xD
Inside, the Seals attack.
Seals (x4):
HP: 7777
MP: 10000
Str: 40
Def: 10
Mag: VARIES, 1-15
MagDef: 20
You can steal four Elixirs if you care to. Each seal guardian has a different elemental absorption. They start off doing a lot of physical attacks, so summon Golem. If you have pretty good mdef, you can then just start attacking them, else start casting Shell for when they switch to magic attacks. Consider having a Blue Mage or Learning equipped to learn Aeroga.
The simplest way to go about this is to summon Titan, find out which one absorbs Earth magic, then attack all of the others with Physicals/Titan. Once they all die, proceed to either use Comet or Physical attacks on the remaining one. Four Monks will !Summon level 4 or four Summoners with Barehanded would be ideal, but you can use many different combinations. Black magic is almost worthless in this battle because of all the elemental absorptions and immunities (Each seal is immune to Wind, Ice, Poison, and Holy, and two of them absorb fire and earth). Time magic is useful for Hasteaga and regen, and White magic with Curaga and Shell. The Earth seal has a nasty surprise though, which is Earthshaker, but you should be floated, so no worries.