There is no easy counter for Magnezone than can be teched into any deck, that's one of the things that makes it so good (along with insane draw power and insane hits.) There are no cards that can easily be teched into any deck to counter magnezone, so you either have to use a deck that's build to beat him, or, imo the better way, use skill. 140 HP usually isn't amazingly hard to kill. Once they drop however many energy to kill your pokemon they aren't as much of a threat. I've been playing magnezone since HGSS-On was announced and in my experience when i send him how to kill something i usually expect the return KO. You can also limit his damage by making your opponent waste their energy drop for the durn, or using disruption to make them miss it altogether. Magnezone can only do 50 dmg a turn, and not even that if they can't drop an energy.