Giratina-O / Tentacruel / Forretress / Ghost Arceus / Kyogre / Dialga
No one rating this?
Straight off, ExtremeKiller looks a bit worrisome, Ghost Arceus is a shaky check with Judgment and Giratina-O is straight up OHKOd by LO Shadow Claw, so watch out. I know it's a matter of getting Leech Seed off and switching around but it's still a bitch.
Darkrai is a problem too, as you only have one Pokemon that outspeeds it and nothing to take a Dark Void.
A Mewtwo with 3 or 4 attacks would prove a threat too, nothing really like switches in on it and it can beat Arceus Ghost if it has CM. Giratina-O shakily checks it, but with a -SDef nature is easily 2HKOd by Ice Beam.
Rayquaza is also a big problem for teams like these, carrying moves that can severely hurt everything and Arceus Ghost not being to OHKO it is a problem as well.
I'd also watch out for Kabutops, whose rain boosted Waterfall 2hkos Arceus Ghost (iirc).
Watch out for Sub Seed Shaymin-S, as it sets up on Ferrothorn and can actually do severe damage with Dialga with a neutral Air Slash if it gets the SDef drop (Expert Belt with Earth Power and HP Fire actually is a bit problem too, but only I use that I think...)
Gliscor can also set up on Ferrothorn, and much easier a Burned Ferrothorn, and be really annoying.
A Cloyster + Wobbuffet can also sweep you clean if you're not careful with Ferrothorn.
A Jolly Bisharp can sweep you clean if it gets a chance to set up.
Aside from all that this is a very nice team, and a good example of rain balance. There isn't much I can suggest that wouldn't change the whole team, but consider:
Jirachi over Tentacruel
Forretress over Ferrothorn
Giratina-O to phys.def or phys.def Giratina-A.
This improves the defensive back bone of the team greatly as Arceus Grass can check non-V-Create Rayquaza with Ice Beam, Jirachi can take on Mewtwo and most CM Arceus reliably. Forretress can counter Gliscor with HP Ice. Physically defensive Giratina is a very good check to ExtremeKiller.
Dialga can run more speed to get the jump on Jolly Bisharp, but it's rare. Kabutops is also nicely checked by Grass Arceus.
cool team, solid for its metagame