Rotom-C@Sitrus Berry
Modest
Levitate
252 HP/252 SpAtk/6 SpDef
Thunder/Thunderbolt
Leaf Storm
HP Ice/Thunder Wave
Protect
Very simple set here, much like a standard Rotom-W. Thunder takes advantage of its perfect accuracy under Rain, and has boosted power over Thunderbolt. Thunderbolt is only if you are worried enough about opposing weather, the power boost between them allows Thunder some 2HKO's on common threats like Metagross. Leaf Storm is just common sense, deals with Gastrodon. Without HP Ice some Dragons like Latios or Hydreigon will wall you, not having to use Leaf Storm and lower Sp Atk is also important in Garchomp's case. Thunder Wave will make opponents slower, yet Rain Mons tend to be a bit on the fast side anyways. If Rain isn't up lowering the opponent's speed will be a very helpful tool. Protect is Protect.