Venasaur is awesome if you can put a Venasaurite on it (halfs the damage taken from fire and ice moves).
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Should I go with a Tyranitarite, Venusaurite, or Gengarite?
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Tyranitarite just makes Tyranitar...well a better Tyranitar. It doesn't really do that much to be honest. Out of the three you listed I would go for Venasaurite.