I really thought I read the method on this board somewhere, but I couldn't find it to come praise it, so I'll just state it here, but this is an ironclad way to hit all gold medals on single player.
*Reset your characters. Not your levels, just your characters.
*Suffer through the Dark Credits once to unlock Jack of Spades.
*Do not unlock any abilities for anyone on the game board that isn't Jack of Spades. Have Hammer unlock her secret character when possible. (Can't remember his name), then run with that guy, Garth, and Reaver.
Once you have all the outer tiles done for Jack, you can just crush the gold numbers for each level, and since the other people aren't leveled, they don't get in the way too much, and they're all ranged, so they don't get mired in your coins.
I had no problem whatsoever with any of the levels, and 2/3 of them I didn't have the magnet nor extra multiplier. Of course, when I did, super easy.