This has been a really annoying combo for me as I use to rely on Hippodown and Blissey as my defensive core, easily allowing a volt-U turn chain to start. Recently I have been trying out pokemon who can break this combination. One of the most effective I have used is Rotom-C (grass).
Item: Choice Scarf
Nature: Timid
Ev's: 4 def 252 SpA 252 Spe
Moves:
-Volt Switch
-Leaf Storm
-HP Fire
-Trick
With the above set rotom C is a great switch in to any Rotom-W set with only wil o wisp being of slight annoyance, 4x resisting any electric move and 2x resisting water, while being unaffected by trick.
If rotom-w volt switches out, Rotom-C is generally faster than what ever comes in so it can also volt switch out to a counter. If scizor switches in from a volt switch it can be handled with HP fire, while rotom-C can also switch in to bullet punches shifting momentum to your side.
I think it has a set of unique differences which separate it from celebi. Rotom-c has a stab Volt switch to abuse while celebi only has U turn (if that). Rotom-C also fears trick much less than celebi due to carrying the move itself and lastly Rotom-C is able to switc into scizor's bullet bunch due to its resistance while also resisting electric 4x not 2x.
Rotom C is an underrated poke in the OU metagame at the moment but try it out and see that it can function just as good as anything going right now in the current meta.