How do you beat ash with Mewtwo on soul silver?

To defeat Lance's Dragonite with Mewtwo use ice beam. Dragonite is a dual Dragon and Flying type, taking neutral damage from ice type moves. Ice-beam is a powerful single-target ice-type move which will deal significant damage to Dragonite

In addition train Mewtwo to a level comparable to Lance's Pokemon, typically 55-65. Utilize status conditions such as sleep or paralysis if possible.