Drop Lock-On and Hyper Beam.. They waste turns. It would be better to use Tri Attack twice than to use Hyper Beam and have to recharge.
Porygon Z ideas:
1. Give it a choice scarf and use it as a revenge killer.
OR
2. Use Agility instead of Nasty Plot to clean up at the end of the game.
OR
3. Use Tailwind on another Pokemon.
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4. Pass speed to it with Baton Pass Ninjask
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5. Pass Quiver Dance to it from Venemoth
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6. Use light screen and reflect to protect it
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7. Play TeirShift where it gets a stat boost and is thus more viable.
Good moves for Porygon Z: Tri Attack, Dark Pulse, Hidden Power Fighting, Thunderbolt, Ice Beam, Nasty Plot, Agility.
Dark Pulse and Hidden Power fighting allow you to get past ghost and steel types which would stop Tri-Attack.