It is possible to run a Mixed set with either a Naive/Naughty nature with moves like Meteor Mash / Psychic / HP Fire / Grass Knot or Bullet Punch. However, since it's hard already to get good IVs for a genderless pokemon, it'll be even harder to get the IV spread needed for HP Fire. And all-out attacker, which is what I run cuz I prefer trying to outspeed everything possible despite the 70 Base Speed, can also work. For that, I would run run Meteor Mash / Zen Headbutt / Bullet Punch / Hammer Arm or Pursuit.
With Rotom, since we don't have the necessary TMs to utilize forms like Rotom-W effectivetly as a bulky mon like used in XY/ORAS OU, you're better off either running a bulky attacker with mainly HP and Speed investment and the rest going to Special Attack or run just an all out attacker Rotom. Movewise, that's really tough since Rotom is really limited. Depending on what form you choose, the three other slots would go to Discharge / Shadow Ball or Hex / Thunder Wave or Confuse Ray.