Okay, Celebi is a very unique type of Pokemon that should never be used offensively. It's perfect role is a healer/support. Anyway, here's how you really maximise it's potential.
-Recover - for obvious reasons.
-Baton Pass - to pass on all stat boosts and activate Natural Cure ability.
-Calm Mind - Can be used to defend Celebi and/or strengthen the next Pokemon through Baton Pass
-Ancient Power - Raises all stats, for baton pass
Basically, keep Celebi alive with Recover for as long as you can, while raising your other stats with Ancient Power and Calm Mind. When things look dire or you've raised your stats high enough, Baton Pass out to the next Pokemon who is subsequently overpowered.