This is it... Time to pull out all the breaks.

Use Hypnosis as well, making that big Dragon pass out. If its out, Dream Eat it away.

If Giratina is still awake, then Mimic the Shadow Force, use it right after it does, then hit it with an Ice Beam.

If Giratina faints before it can use it's last move, then use an Aura Sphere on the Darkrai.

Hypnosis ~Dream Eater~Dream Eater (Subs: Mimic~Shadow Force~Ice Beam or Mimic~Shadow Force~Aura Sphere)