Shadow Ball/Thunderbolt/Hidden Power (if it's Fire or Ice, check in the game corner exchange place on Platinum)/Focus Blast/Trick (if you have a choice scarf) or explosion (if you wanna run life orb).
Also, EV it in Special Attack and Speed, and give it 4 Defence EVs.
But really, this is the wrong section, your should go to strategies and movesets
Go for explosion/trick if you hidden power isn't fire or ice