I'm going to assume you are trying to get the Light Ball. Easy. Get a Pokemon over 231 Speed with Thief, and use Thief on Pikachu before destroys you. Fast Pokemon that you can use is Aerodactyl and Ninjask. Ninjask is perfect, it can use Protect to get a Speed Boost, and steal the Light Ball.
If you used you TM Thief on Gengar, then you must train it to a higher Level.