Xenoblade Chronicles/Tephra Cave

Provided you finish with the sidequests, you should skip to Mag Well Ruins. Going through a number of cutscenes, you'll encounter small spiders to contend with. Before you get to Villa Lake, Reyn gets captured by a spiderweb and you'll be on your own temporarily.

Go up the vines and follow the path, you'll encounter a scenario that future vision plays a role for the first time and unlocks Monado's Shield which block arts easily. So use it against the giant spider which should be on the easy level. Use the item to unblock the web and escape. Most of the area in the cave is further unlocked. The rest will be unblocked near the end of the game.

You'll get the chance to see the Mechonis Giant from the angle out on the other side. Go up the vines and get to the exit to reach Bionis Leg.