Tiamat, the boss is level 25. You should be 25 or higher to cross the mines "comfortably" but not too overleveled.

Tiamat Stats
* Level: 25
* HP: 49,993
* MP: 999
* Steal: Hi Potion, Ring Wyrm scale, Leo Gem
* Air eater -1/2 damage on all other elements
* Weak: Earth