For the Sunstone:
Trade for three Ruby Plates in Prehistory. The Red Vest is even better if you have one from a sealed treasure chest, since it converts fire damage to health. When fighting the boss, keep hitting the small orbs until you find one that transfers the damage to the boss. Slam on that orb until he shuffles them, and then just find it again.
Another side-quest you absolutely should not miss:
Talk to Balthasar in the dome near Death Peak and do what he says.