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WiFi/Wireless battles are the same thing, just different connections. Either way, you are battling against a human and not a computer.
Since you and your friends will be using Ubers, you'll need to prepare your Mewtwo to beat other Ubers that your friends may use.
First, learn to EV train. That will make Mewtwo and your other Pokemon much stronger.
Getting good items is also important. A Mewtwo works best with Life Orb for power or Leftovers for healing.
Use Thunder instead of Thunderbolt. Since Kyogre is allowed, you can use the permanent rain to boost Thunder's accuracy. If you or your friends do use Kyogre/Rain Dance, then use Aura Sphere over Flamethrower since it will be weakened. Aura Sphere is also supereffective against more Pokemon than Flamethrower.
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