Something that has always confused me about the relationship between Darkrai and Cresselia is that according to the lore, Cresselia is supposed to be able to counter Darkrai and prevent nightmares, but the game mechanics cleary show differently. What with Darkrai being immune to just about every attack and able to put Cresselia to sleep.

Here we find Celine the Cresselia finding an alternate use for psychic. Dark types might be immune to psychic attacks, but that doesn't mean the environment can't be taken advantage of using a conveniently placed water hose.

Can't give anyone nightmares if you can't move.