The team looks great; however, it really lacks offensive pressure required to keep up with rain or tailwind teams.
Cresselia also is extremely status-prone by not carrying rest OR lum berry. Sure, it will wall an inexperienced battler to hell, but a paralyzed, poisoned or sleeping Cresselia would hurt you a lot as it is your most important mon. I'm thinking ChestoRest would be a better fit for this team.
I'm also thinking Gyro Ball>Power Whip on Ferrothorn as you will want to be Leech Seeding over Power Whip in most occasions.
I think that Tyranitar>Terrakion for what you want to use him for. Just reading your description of Terrakion made it seem like you would want Tyranitar instead. Unnerve is also much more useful in doubles than singles or you could be bold and go dual weather.