Use psyhcic and dark and ghost attacks.

Dark crater is mainly fire-types, but there are also ground and 1 rock-type, Rhyperior. I
would suggest water-type attacks or Dig.
Darkrai won´t be a problem to hit if stun him with a Stun Seed. So he can´t use Double
Team until you beat the others
Keep an eye on Cresselia. She may use up a lot of Reviver Seeds...

use the golden seed cheats and share them eqaully with your partners you should be a high
anough level to defeat darkria you should also know a veraity of moves