DP Magnezone's Magnetize refers to your active pokemon, which can be anything as long as it's got a Metal Energy attached. Metal doesn't even have to be basic or special metal. It could be a Double Rainbow Energy, for instance. It could be a Scramble Energy when you're behind on prizes. As long as it is currently capable of providing Metal Energy, Magnetize can give you free retreat. (IE: Multi and Holon FF on the same pokemon won't give Free Retreat, 'cause the Multi won't be able to provide metal in this instance)
Funny to note that it doesn't lower your bench's retreat cost... Stuff that damages benched pokemon without a retreat cost won't get help from Magnezone (unlike Phoebe's Stadium, which does not limit it's boons to only the active pokemon)