If you can correctly anticipate that the opponent is about to switch in Ditto, and your current Pokemon is faster than theirs so that you go first, you can really give Ditto a nasty surprise by switching to a Magnezone with Magnet Pull, Protect, Substitute, Toxic, and Explosion. That will force Ditto to copy that, it won't be able to switch out because of Magnet Pull, and its moves will have fewer PP than yours so it will be forced to blow itself up with Explosion first; its other moves don't do anything against you. Then you'll be able to get a Toxic on whatever Pokemon they send out next from behind the safety of a substitute, and/or use Explosion for however much damage that deals, to turn a net profit after trading for their Ditto plus however much of the next Pokemon's health you can wear down.
Granted, this isn't a good competitive set, and it's almost completely worthless when it doesn't get to face a Ditto who copies it. But when it does...that's what you're looking for, right?
