blitzlefan
shake it off!
My argument: Dialga is a check, and checks often involve a little bit of risk, but Dialga still does well at checking Arceus-Ghost, either wasting its boosts and making it suffer entry hazard damgage, or paralyzing it for the rest of the match. Both render Arceus-Ghost a lot easier to kill later on. Yes, that's true, but it also requires that they hit with Focus Blast twice in a row; there is a 49% chance of this happening. However, 39.414% of the 15.02644% of Arceus-Ghosts carry Focus Blast, so the chances of this successfully happening is approximately 3%. (This part isn't my argument but it might make us feel a bit better.) Also, within our final two slots, we can run a backup check to Arceus-Ghost if we still feel shaky about it, either a faster team member or a scarfer that can deal heavy damage.
A second idea: Darkrai. I don't really know why it's been dismissed, because Darkrai can put Arceus-Ghost to sleep and that's the end of it. We've established that SD Arceus-Ghost is underwhelming and less common, and ExtremeSpeed does not KO, so Darkrai can get off a Dark Void if needed. Also, unboosted Dark Pulse does tons of damage to Arceus-Ghost.
Lastly, we can just Toxic stall it to deal with Tentacruel or something lol.
A second idea: Darkrai. I don't really know why it's been dismissed, because Darkrai can put Arceus-Ghost to sleep and that's the end of it. We've established that SD Arceus-Ghost is underwhelming and less common, and ExtremeSpeed does not KO, so Darkrai can get off a Dark Void if needed. Also, unboosted Dark Pulse does tons of damage to Arceus-Ghost.
Lastly, we can just Toxic stall it to deal with Tentacruel or something lol.