Fan form is the worst of all the Rotoms because he has a redundant ability come on Levitate on flying type geez anyway here is my set.
Fan-flinch
Nature: Timid/Modest
Ability: ugh...levitate
EVs: 252 Speed, 252 S.attack, 4 hp
Item: Air-balloon...just kidding. Leftovers
Thunder-wave: Para
Air-slash: Flinch + STAB
Volt-switch: STAB+Switch
Shadow-ball: Coverage
Also you could switch Thunder-wave for Will-o-wisp and Volt-switch for Discharge if you want to damage for the paralysis.