Back in the day I attempted the Ghelsba fight once and... died. Neither I nor the other 2 ppl really knew what we were doin. But heres how id imagine the fight would go:
First, ya need 20 Beastmen's Seals. These are Rare/Ex, & can drop only offa mobs ya get exp from. Trade em to a guy in Poet Jeuno (dun remember his name) & you'll get an orb. Next go ta the circle in Ghelsba wit 2 other people & trade the orb; ya & your 2 friends will be able ta enter. The fight is against 3 bats: 1 Colo-colo & 2 Furies.
Party setup: WAR, BLM, BRD. BLM + BRD should have juices & pies, WAR uses mithkabobs.
BRD casts Minne + Paeon II on herself & the BLM, & Madrigal + Minuet on the WAR. The WAR runs in, aggros the 3 bats. BLM (who should have a capped Enfeeble skill) Elemental Seal + Sleeps 1 Furies, & the BRD (who should have Marys Horn + a capped Wind instrument skill) Foe Lull's the other Furies. The WAR then 2-hr's (Mighty Misses!) & procedes ta take on the Colo-colo himself while the BLM + BRD keep the Furies asleep. Both Cure themselves & the WAR when needed, & use juices. The BRD should also keep Minne + Paeon up on her & the BLM.
After the Colo-colo is dead the WAR pulls aggro off the BLM's Furies. BLM Manafont + nukes, BRD keeps sleepin the other Furies & healing herself. Then repeat process wit the 2nd Furies.
Keep in mind, tho, that this strategy is purely theoretical. Ive neva tried it myself. So use caution :)