I started off writing about Mega Sceptile, but stumbled across this bad boy instead:
Scizor@ Life Orb/Scizorite
Trait: Technician (if possible, if not opt for Swarm)
EVs: 252 HP/252 Atk/4 Def
- Bullet Punch
- Iron Head
Scizor forms a fantastic core with Rotom-W, making good use of VolTurn. Bullet Punch patches up Sciz's pitiful Speed, while Iron Head provides powerful Fairy-killing power to assist Salamence. U-Turn gives useful Bug STAB, and pairs great with Volt Switch. Superpower provides coverage, but Roost heals off Life Orb recoil. Ability-wise, if possible go for Technician, but if you can't get one then go for Swarm. For it's held item, Life Orb gives Scizor the ability to wrecks things more than it already does and works well with Roost. If your bro isn't running Mega Salamence, then I recommend using the Scizorite for boosted Attack and guaranteed Technician.
Scizor benefits greatly from Rain, weakening his glaring Fire weakness. When a Grass-type appears to threaten Rotom, switch to Sciz. He laughs at any Grass attack not named Solar Beam and hits them back hard with U-Turn. In turn, Rotom can safely switch in to incoming Earthquakes. These two Pokémon possess nearly perfect synergy, and Scizor also helps crush threats to some of your partner's Pokémon. Good luck with your team!