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With the sun up it can do big damage to mons weak to it's main stab, solarbeam. Of course, it will probably be predicted which means you need to revenge kill.
I guess a general set would be:
252 SpA/252 Spd/4 HP
Chlorphyll/Timid/Life Orb
SolarBeam
Giga Drain/Leaf Storm
HP Fire
Earth Power
You obviously need Ninetails for this set to work.
Another option for the item could be Focus Sash, though you'd probably need a Spinner.