I would ask FC friends/LS friends to at least make an 8-man group off the bat. Then you only have to wait for 16 others.

If that fails/not possible, I can suggest making a PF party for it, stating you need kind people to unlock or those that need it as well.

Else prehaps Yoshi has plans to re-vamp it so new players can get it, AKA giving 500 myth per run.. to those that have completed it once..

Good Luck.

EDIT: been beaten by synestr, sorry for late response.