Fly Flammie to the northwestern-most red dot which is the Labyrinth of Ice
Walls (where you picked up Undine earlier.) Land Flammie here and you'll end
up outside the cave where the Mana Stone used to be. You'll find the merchant
cats outside here so buy the weapons and amour if you didn't do so in the
desert earlier on. Head east twice to save at the Gold Statue and make sure
your team is L25 or above (L30 or above is STRONGLY recommended) and then
head inside the cave. This place is called a labyrinth for good reason, it's
a real pain getting through this place. There is only one Gold Statue and
it's a pain getting too it. Once you get there, you can level up quite
easily. Watch out for the Posiedons, their bubble attack lowers your attack
level so you only do 1HP damage.

Follow these instructions CAREFULLY, it's VERY easy to get completely lost in
here. To get to the Gold Statue, go north into the labyrinth so that you're
now in the cave where the Mana Stone used to be. Now go northeast into the
main labyrinth (last time you came here, the labyrinth entrance was sealed so
you couldn't get in.) Now, go southwest and then north (2nd door.) You'll be
trapped. Beat up all the monsters here and continue north. Go west, north
(1st door), north, northwest, northeast, southeast, northeast and save here
(Gold Statue.) Level up to L30 minimum using the local area and when you're
ready, come back and save again. When you're ready to face the GB, go south
from the Gold Statue, southeast, northeast and prepare to face off against:

***Boss: God-Beast Fiegmund***

Fiegmund is easy, just like Dangaard. Ice melts when exposed to Fire, right?
So use ANY fire move. Fireball, Explode, Flame Saber. It all works. Keep a
few Puipui Grasses and Stardust Herbs handy to cure Freeze status. Just keep
hammering away and he'll die before long. When he starts climbing behind the
platform, use Hawk (Fire Jutsu) or Angela (Fireball/Explode) -- physical
attacks can't hit him from here. Oh, and make sure you've got some Magic
Walnuts handy too!

Once you beat Fiegmund (which shouldn't take too long), you'll wind up
outside, fully healed. If you haven't already done so, fork out on the new
weapons and armour from the cats outside. Next stop, Forest of Wonder.