I have a masked man team and i can tell you its really annoying when someone has genjutsu mirror.

Other than that there is not much you can do, you wont kill him in turn 1 and you won't interrupt his mystery so all you can do is endure and reflect the status back.

Masked man is not a DPS ninja so don't focus on him, its better to kill the combo linker or DPS ninja in the opposing formation. Besides he can only use his mystery once every 3 turns.