Hmm, finished it today. Quite a hard dungeon.
Use something that'll stop Darkrai, like a sleep seed, etc.
Take out the pokemon you have a type advantage over.
Be aware of Cresselia, its darn weak, I've fallen a few times there just because of her.
Also, for the kicks, get a REVISER seed to Darkrai, it WILL be funny.
A full-area of effect should be very useful here. Like your Squirtle's Blizzard. You know how to get him to use Blizzard only, right? If not, go to 'moves' and press >, untick the star mark from all moves other than Blizzard and set his IQ skills to include 'exclusive move user'