Equip before leaving
Open the character menu and set up the ARC. Check each party member rather than only the lead character. Repeat this after obtaining the third crystal.
Prepare the party, cross the glen and return from the mine.

Open the character menu and set up the ARC. Check each party member rather than only the lead character. Repeat this after obtaining the third crystal.
The path to Glen Dewy runs east through the forest. Walk or run towards narrow edges to jump. Look for ropes and other usable scenery before assuming a route is blocked.
The warning before the boss is a useful stopping point: equip newly found gear and crystals first. Keep recovery available rather than entering with a depleted party.
After retrieving the crystal, leave the mine and return home. The journey changes after dark, so do not treat the outward route as a promise of an uneventful return.