Zodiark eh?
First get all espers, then beat the hunt that all the Garif's give you.
After that hunt go the the Garif Geomancer (or whatever the name is) he will thank you for killing that monster and also tell you that the path in the mines is now open.
Then go the place where you battle Mjrn(you know that Viera) and you will see a route to Zodiark.
But be aware that the enemies are VERY powerful.