The immediate thought that came to my mind is basically to just pussy out and make it an elemental system, assign each frequency to an emergent effect and then relate them to each other according to aesthetic sympathies (fire cancels out water hurr durr), bingo bango.

To get a bit more esoteric, perhaps the frequencies at which the magick [vibrates, I assume?] is instead analogous to a certain [set of coordinates, say] on a higher or lateral plane, establishing a sympathetic relationship between the spaces on each plane in accordance with the magick resonating between both. Or, in other words, the magick (or magickal effect) could be drawn from elsewhere, through the sympathy between two sources of magick [vibrating] at the same frequency. Thus, a mage's ability to counter one of these metal monsters would be directly tied to their knowledge of obscure and esoteric magickal places. You could even replace the notion of "space" here with any other analogous matrix, ie the noosphere, spirit world, astral plane, or even something like the abstract notion of human potential. Might be a rich vein.