I personally don't really care for 100 myth. You are not doing these fights for Myth, you want the loot at the end.

One thing that would absolutely give most people the incentive to teach or take on someone that has not cleared would be to have this bonus of 100 myth(soldiery in 2.4 most likely) replaced with "greatly increased drop rate of items" such as mounts, mirrors, Ramuh weapons those items that don't drop 100% of the time and also the craft items for bardings and furnishings.

Also for fights that do not have additional loot that don't always drop, like Coil for example. For Coil you could have the chance for additional armor drops from chests.

This is such a simple solution and it's only a "chance" you can't exploit it and most of all, everyone will want the bonus that the person that hasn't cleared gives you.