I have this quest the game won't let me complete.

[37491526936363]

I have been playing this game for a while now (I'm on level 70) And ever since I reached level 40( maybe), It won't ever let me finish the quest. To be more specfic, whenever I click the quest, it takes me to Mount. The quest is Strict Erza and to complete it, it says I need to get a black dragon's scale. Do I need to complete the quest, or is this a glitch in the game?
I would also like to add that I have battled the world boss multiple times. Do I need to defeat him in order to get a black dragons scale?

[fairycana]

Hey, I haven't completed it either but I think the last hit on the World Boss event gets the black dragon scale.
From screen messages I'd say you need to defeat an opponent in Pandemonium for the beast bone (correct me if I'm wrong).
And the Trophy of the strong is shown in the Arena Rewards Achieve section for placing in the Top 90.