Our Xenoblade Chronicles guide features a comprehensive walkthrough and additional info on the endless amount of optional content in the game.
Walkthrough: Tephra Cavern
You need to make your way through Tephra Hill/Cave to reach the Mag Mell Ruins, and then continue through a new section of the cave to get to the Bionis’ knee. The enemies up to the cave are not worth the time it takes to fight them. The Wood Bunnits, Prom Skeeters, and Caterpiles only net you 1-5 EXP. You can take on the UM, Evil Rhangot, located at the bend in the hill, which you were likely too under-leveled to defeat earlier. The Monado and your increased levels give you the advantage this time around. There’s not much else to consider before entering the cave, so let’s do that.
When you reach the ruins, head through the recently opened door to enter the Escape Pod Bay. Take note of the ether lamp nearby; they’ll guide you through the cave on your way to the Bionis’ Knee. Just past the Escape Pod Bay is the Tephra Cavern. Along the outer path is a Ice Ether Deposit, which you can mine for Crystals. Drop into the cavern and if you wish, battle some of the Desert Krabble, then take on the Noble Brog along the eastern wall. Follow the ether lamps towards the southern exit. You’ll come across a few Worker Arachno as your approach the next goal marker.
After defeating the Arachno, Reyn and Shulk will return the dead colonists nearby to the Bionis as a form of burial rite. Once you regain control of the duo, you’ll be in combat against roughly eight Worker Arachno and a Director Arachno. It’s not a difficult battle, but rather, a taste of things to come. Head west from there and you’ll come across an Arachno pit. Rather quickly, Reyn will get ambushed with webbing and hauled up into the ceiling and you'll have to come to his rescue. Head for the southern exit.
You’ll enter Vilia Lake. Your goal is the pillar covered in vines just south from your position. (There is another quest related item to the north, the Lost Pendant.) There are some new enemies that you can take on, but avoid the Gluttenous Eugen for now; Shulk can’t defeat him solo. Before climbing up the vine, check out the waterfalls to the south. One of them veils a Water Ether Deposit ripe for mining. After that, climb up the vines and head northwest towards the exit. After a cut-scene, you’ll be thrust in to battle with the Arachno Queen.
The Monado & Visions of the Future
Thankfully, the Monado can provide visions of the possible future during battle, and tip you off to incoming attacks. Take note of the Vision Tag at the top of the screen during battle. It shows you the incoming attack, the intended target, and the result if you are unable to thwart the attack. You’ll also see a gauge reflecting the amount of time before the enemy will unleash their attack, so react/plan accordingly. Part of this planning includes “warning” your teammate(s) by standing nearby and pressing B.
Individual font colors reflect different types of incoming attacks: White > Talent Arts, Red > Physical Arts, Blue > Ether Arts
In order to block a Talent Art, the Shield Art is required. You also need to have a full segment of the PG charged at the time of the attack. The Shield Art must also be at the same level of the enemy’s Talent Art in order to be effective. For example, Arachno Crush I requires that Shulk’s Shield Art be at least at level 1.
Use Shulk’s Sweep Strike to damage as many Soldier Arachnos as possible. When the Monado is charged, use the Buster attack on the Queen. Remember; keep an eye out for her Talent Art (you’ll get quite a bit of warning) Make sure to heal Shulk or Reyn if their health dips below half, just in case you get overwhelmed.
With the Queen dispatched head northwest. Remove the webs blocking your path by simply pressing A. At the end of the path, you’ll exit the cave and get your first glimpse of the scale and grandeur of the Bionis and Mechonis, the two beings who comprise your "world".
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.