Our Xenoblade Chronicles guide features a comprehensive walkthrough and additional info on the endless amount of optional content in the game.
Walkthrough: Ether Mine / Central Pit
Welcome to the Central Pit. Take a ride on the elevator that's north from your starting position. Run towards the machine in the middle of the pit and follow the trail clockwise, being careful to avoid the gigantic rotating fins along the way. Follow the first path you come across, breaking off towards the east. Take a left at the end of the bridge and take the next elevator (Personal Lift 2) down to the Central Pit - Level 2 .
Work your way down the slopes and back onto the large device in the middle of the pit. Head south, continuing clockwise, until you reach an elevator-platform. Hop on board when it's heading up, and hop off on the next level. Take the path clockwise and ride the next elevator down a level. Run north, back towards the cliffs lining the pit. Head as far southeast from there as you can and you'll find Personal Lift 3.
The trick to continuing from here is to ride the rotating fins, one by one, until you reach the other side of the pit. Your first stop is the northern platform. When you arrive, a level 24 M84 Mechon will drop down from the machine. Once he's laid to waste, hop back on the fine towards the next platform where you'll face off against two Mechon M84. Once they're finished, go for the tunnel leading away from the pit. Before hopping on the elevator, check out arts, skills, and equipment; there's a boss fight immediately after your upcoming elevator ride.
Boss: Xord, Level 25
The trick to defeating Xord is to utilize Chain Attacks to inflict Break; attempting to debuff him with Break any other way is destined to fail. It really helps if you can fill up the Party Gauge ahead of time. You can warp to an earlier part in the Mine if you'd like to do that. Otherwise, the weaker Mechon Xord summons make for excellent fodder in the race to recharge the Party Gauge, so have at 'em. You don't need to deplete all of his health at the moment. Once he loses about a third, the fight will conclude…for now.
After the cutscene(s) head up the hill and get on the elevator. Continue through the tunnel and head up the stairs. You want to make your way towards the freight elevator to the south, by way of the large rotating fins. Once you try to activate the elevator, Xord will reappear to finish the fight, so prepare yourself.
Xord is much easier to defeat in his weakened state. Enchant your party and Break/Topple him as early as possible. He won't summon any minor-Mechon, so you can focus on him the entire fight.
There is an apparent mistake in Special Lever 2 for Regbuilding Colony 6. It says defeat Earth Nebula at Sap Cave. I was never able to find a Sap Cave, but I was able to defeat Caris Nebula at the brog lake just before you get to Nopon Arch.