Okay, so that means we're working with base stats.
Obviously, gonna need some high base speed, def, and spc def. I really don't like doing the number crunching so I'm not gonna suggest any specific numbers, but I'd say speed needs to be highes with Def and Spc Def being secondary. A design would be nice to help assign which of the two defenses may be offset, but we don't have that atm.
Other out of ordered thoughts-
this pokemon needs these moves:
Reflect/Light screen- obviously to up Tanky-ness
Iron Defense
Curse- stalling for curse to kill or force switch
Haze- ghostly vapor reseting stat boosts is totally legit.
Night Shade- Like blissey/chansey, has a good move to use that doesn't rely on its terrible atk stats but still has immunity to worry about
Rock Polish- the fast keep getting faster, as usual.
Magic Coat- fits steel/ghost perfectly as does v
Mirror Coat
Bide
Perish Song
Taunt
Recover- perhaps
Spikes
Doom Desire
Mirror Shot
Flash Cannon