Regarding the life of Fisher Tiger in One Piece
First, climbing a mountain couple thousand meters high isn't a big performance in the one-piece world. Luffy climbed a 3000m (~9000 feet) high mountain covered in ice and snow when he was on the drum island before the timeskip (Before getting stronger).

Second, Mariejois is the land of the descendents of the gods, the celestial dragon's. Celestial dragons hate every kind of person who's beneath them, even marines, so it's probably not very armed, Marines only use it as way to the New World and they still need to wait months to get access.


  The Holy Land Mariejois (聖地マリージョア Seichi Marījoa?) is the capital of the World Government and functions as the primary path which individuals seeking to enter the other, far more tumultuous and dangerous side of the Grand Line, the New World, pass through.


And third, Fisher Tiger only attacked the home of the celestial dragons, where the biggest defence are the slaves.