This Pokemon cannot be made to flinch.

Does mental Herb stop fake out?

It will block Fake Out and Prankster status moves allowing Cresselia to set up. If that doesn't work then give Cresselia a mental herb.

What does the mental herb do?

The Mental Herb (Japanese: メンタルハーブ Mental Herb) is a type of held item introduced in Generation III. It cures the holder of infatuation, and from Generation V onward, other mental effects.