Lol @ this. Hmmm, how about:
Steelix @ Occa Berry
Rock Head
Evs: 252 Special defense / 100 Atk / 156 Hp
Careful
-Earthquake
-Stealth Rock
-Roar
-Explosion
This might not be the best counter, and I haven't run any calculations so I'm not positive of the ev's either, but I decided to try out a less used pokemon ^^. With the highest possible amount of sp def, and a good bit of hp evs combined with the occa berry it should be able to take the hp fire from magnezone. No other move really threatens it, and with the 100 atk evs, EQ should do decent damage (I will have 200 attack and EQ will end up with 600 base power) If all else fails I can always Roar him out. Btw this beats both magnezones XD. So yea, this is not the best counter in the world, but I wanted to use a UU/BL pokemon.