The eight God-Beasts are the monsters which destroyed the world. The Goddess
of Mana used the Sword and defeated all eight. She imprisoned them in the
Mana Stones and the Stones were protected by the Spirits we have already
obtained. The God-Beasts are unimaginably tough and you'll have your work cut
out for sure.
You can defeat the God-Beasts in any order, but I find that beating them in
the order of this walkthrough is the easiest, as the enemies and bosses along
the way get higher and harder.
The God-Beasts only know attacks that are taught by the relevant spirit so
Xan Bie knows Flame Saber, Fireball, Explode, etc. But doesn't know Holy Ball
or Dark Force as these aren't taught by Salamando.