As in to include in your Darkrai deck or against Darkrai? If the latter, Claydol with a pluspower OHKOs Darkrai Lv.X and it doubles as a great support Poke.
Wigglytuff GE with Sidney's Stadium - Put both Pokemon to Sleep but Sidney's Stadium prevent Darkrai from Sleeping and also max out Darkrai's Sleep Hole to 80 if defending Pokemon is a Sleep