How? I can’t seem to figure it out.
if u defeated the quest bosses, nope
but u can fight it again if u party with someone who has the quest and share quest
but u wont get the quest items
but how do I get certain materials then? I need pure crystals from the mines bosses, but they are untradeable, and I can’t re fight them… so what now?
Just keep doing quest till you finish all quest in mine. You should get all the material unless you didn’t pick it up when it drop. 
I did them all, and thought I picked up everything. Guess I’ll just never have the materials then 