Here are the damage amounts:
attack- min damage-max damage
Pursuit (non-switch): 82-96 HP
Pursuit (Switching) 158-186 HP
Crunch-158-186 HP
These all assume you have no item, moxie boosts, critical hits or defense drops from Crunch, so keep that in mind. The goal is to get 206.5 HP~207 HP of damage, and with max attack, (366) you can do about 174-204 damage to this Cresselia (using crunch), meaning at the very best, it still has about 3 HP to spare, so you can't OHKO it without changing something like adding an item. If you happened to get a critical hit, moxie boost, or defense drop from Crunch, you could take it out, but obviously, you're not trying to depend on luck here.