For a moveset, I'd have to say:
- Thunderbolt
- HP [Ice]
- Magnet Rise
- Thunder Wave/Flash Cannon/filler
Steel isn't the best attacking type, but Flash Cannon could do. Thunder Wave is a better team support role. HP [Ice] is almost required on this bad boy, because with Flash Cannon being his only other move, something Garchomp or Swampert will easily walk all over him.
Anywho, make him Timid, and max out Special Attack and Speed with EVs, with any remaining in HP. Now, why the Timid nature and maxed Speed, you may ask? Well... Magnezone is Skarmory's worst nightmare. And with Timid and maxed Speed, he then also becomes a Metagross killer, since an absolutely maxed Speed Magnezone will ALWAYS outrun Adamant Metagross (barring the use of Choice Scarf and Agility), so if you switch in on a Meteor Mash, you can fire off a Magnet Rise before Metagross has the chance to Earthquake you. And then kill away. The only thing you have to look out for is Explosion and, to a lesser degree, Hammer Arm.