How do I beat Champion Lance in Pokemon Heart Gold?

Get rid of ditto for something else. Ditto isn't all that useful and you'd be better off with another pokemon, especially one that can learn ice beam. If you have an ice beam TM you may want to try using Vaporeon; it is bulky and has good special oriented offenses. Entei, Gyarados, and Donphan need to be trained to higher levels (I'd say at least 45 but you'd be better off training them to level 50). Gyarados needs to know ice fang; it will be super effective against many of Lance's pokemon.