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Darmanitan isn't that great in OU, he's better in UU, but you can use him if you want.
Either way, you want to kill all the bulky water types and stupid shit like Gliscor, Landorus, Thundurus (who can wreck your sweep), and Priority abusers who can fuck up your Darmanitan's crappy bulk, like Talonflame and Azumarill. Honestly, be hind of sucks and need a ton of support, now that I think about it.
Nasty Plot Celebi is good for that, and does a decent job shitting on other things too. As for stupid shit like Gliscor, I'd use Black Kyurem since he can bust up the enemy team a bit and soften them up for your Darmanitan sweep. You need somebody who can get rocks up too, so you don't get screwed over by random sash guys.
Here's a couple good sets for Kyurem and Celebi:
Celebi @ Leftovers
Ability: Natural Cure
EVs: 220 HP / 252 SpA / 36 Spe
Modest Nature
IVs: 0 Atk
- Nasty Plot
- Giga Drain
- Psychic
- Earth Power
Kyurem-Black @ Life Orb
Ability: Teravolt
EVs: 56 Atk / 252 SpA / 200 Spe
Mild Nature
- Earth Power
- Ice Beam
- Fusion Bolt
- Outrage
As for rocks, you can go with any of the usual suspects to set them. most people like Tyranitar or Skarmory, but there are lots of options. I miss Speed Deoxys.