You could use Snorlax. He's got great special bulk, and he can sponge Mega-Manectric's attacks.
Snorlax @ Leftovers
Ability: Immunity
EVs: 252 Atk / 4 Def / 252 SpD
Careful Nature
- Earthquake
- Pursuit
- Zen Headbutt
- Curse
EarthQuake, with the given EVs, should 2HKO Manectric. Pursuit is to hurt him if he tries to switch out, which is probable. (I'm pretty sure pursuit works with volt switch) Zen headbutt is coverage. You could use body slam, though. Curse if your opponent gives you time to set up.
Still, he won't last long without rest, his main recovery move.
You could therefore use a RestTalk Set!
(here's one, if you've never heard of it)
Snorlax @ Leftovers
Ability: Immunity
EVs: 88 HP / 176 Atk / 132 Def / 112 SpD
Adamant Nature
- Rest
- Sleep Talk
- Double-Edge
- Earthquake