I nabbed mine with the following:
Dead synchronizer in the front (the bed must have a lot of uncomfortable springs because it doesn't heal you)
Smeargle 1: L64 or 68 (use 64 if HP IV < 14)
Bold, 252 Def, enough Speed to hit 160, rest in HP
Spore
Taunt
Seismic Toss
Trick
@Leppa Berry
Smeargle 2: L100
Bold, 252 HP, 252 Def
Spore
Substitute
Foresight
Trick
@Leppa
Put the Darkrai to sleep first, keep it under Taunt while weakening it, and send a Leppa Berry its way before switching out.
Smeargle 2 can use Foresight to negate its Double Teams as well as Substitute to block Hypnosis/Nightmare. Substitute is annoying to Sketch because Sketch will fail against a Sub, so you must make sure the Sub user has less than 25% HP so it's too weak to make a Sub.
If you're stuck on Newmoon Island without the right supplies, confront Darkrai and RUN! You can repeat the event once you're ready.
For folks who want to hatch eggs on Newmoon Island after Darkrai is caught, the sailor guy will add Newmoon Island to his sailing locations.