Hmmm...how bouts this:
Magnezone @ Life Orb
Naive Nature
EVs: 252 SpAtk, 252 Speed, 6 HP
- Thunderbolt
- HP[Ice]/Flash Cannon
- Magnet Rise
- Explosion
Classic Skarmory killer. Just switch in, Thunderbolt, switch out. But... with totally maxed Speed, Magnezone will ALWAYS outrun Adamant Metagross. So unless Metagross has an Agility under his belt (or Choice Scarf, which in that case you can predict well enough), Magnezone is free to switch into Meteor Mash or Thunderpunch (basically anything but Earthquake). Once Metagross is trapped, Magnet Rise before Metagross has a chance to fire off an Earthquake, and continue to pwn. Now Metagross has another thing to fear.
You also may be looking at the natures a bit funny... but not really. After having come against a Magnezone, it Exploded on my Blissey, and she survived by a hair. I assumed it was a Timid Magnezone. That increase in Attack that would otherwise be lost with a -Atk nature is greatly welcomed with Explosion (and with Life Orb to back it up, it's pretty much a guarantee that it would OHKO Blissey). So if you go with Explosion, choose Naive (preferred, since his typing leaves him with a boat load of resistances anyway) or Hasty. If you drop Explosion for, say, Thunder Wave, choose Timid nature.
But the moveset I made up, along with the unconventional nature) is virtually a guarantee (if you can predict well) of being able to kill 1.) Skarmory, 2.) Metagross (and any other similar Steel types), and 3.) Blissey.