In VGC, Cresselia should have:
Luna the Cresselia @ Sitrus Berry
Calm Nature. 252 HP, 136 def, 120 sp. def
Icy Wind/Psyshock/Ice Beam
Trick Room/some other filler
Trick Room allow your slow pokemon to outspeed faster pokes. In fact, your team is in an awkward speed tier. Awkward meaning that you can't outspeed fast threats and aren't slow enough to completely abuse Trick Room. Use this to your advantage. If the opposing team is a Trick Room team with tons of slow pokemon, don't use Trick Room and attack. If they're faster, use Trick Room to outspeed. The first attack is for attacking, Moonlight to heal, and Helping Hand provided a nice boost to your attackers.