Some people have a complete misunderstanding of what the mentor role is. As others said, expecting a mentor to know every single mechanic of every single fight, that can come up on the roulette, is completely unreasonable. Square's mentor expectations:

Provide gameplay advice for new adventurers and other players.
Be an exemplar for player etiquette.
Invite new adventurers to the Novice Network, and answer their queries in the chat channel.

That's it. Stop expecting mentors to be a walking encyclopedia of all content. Now, if you want to complain about mentors who have a bad attitude, that's a different complaint.