Porygon-Z used the same tactics from Diamond and Pearl and Black and White: Agility Sweeper or Nasty Plot Sweeper. However, unlike those games, Porygon-Z lacks power in its BoltBeam Coverage (Icy Wind and Discharge are the best moves to use). Although, this is supplemented by new coverage in Diamondblast and Hydro Pump. Also, unlike the canon games, the Virtual Pokémon only has Trace similar to Porygon2, allowing it to counterattack and scout opponents with their own abilities.
The most substantial change for Porygon-Z in Vega is the effect it gains for holding its family's signature item, the Upgrade; this item is now a glorified Silk Scarf, boosting all Normal-type moves to a whopping 60%, before STAB, and grants Porygon-Z the honor of being one of the deadliest Special Attackers in the game. No wonder the devs did not bother giving Porygon-Z Adaptability! Unfortunately, the only way to get the Upgrade is through hacking. (the Gym Leader Avery has this item on his own Porygon-Z, but the only notable Pokémon that have Covet or Thief are Tanuking and Liedoro, who are only available in the postgame.)
Overall, Porygon-Z is a double-edged sword. It is not as powerful as in the actual games, thanks to the nerf in its BoltBeam coverage, but it still wreaks havoc by having the most powerful STAB out of all the Pokémon in this region.