http://www.eyesonff.com/ff6/maps/ff6...in-siggied.png

Use that map to find it. Land your airship in the middle of the star-shaped mountain range on the southern continent. It also shouldn't matter what level your other characters are. As long as they are backed up by even one of your level 70+ characters with decent magic then you'll slaughter anything that moves in that cave. You shouldn't have any trouble getting two parties through alive.