I'd rather use Magic Coat on Porygon-Z than Lum Berry, simply because otherwise Roserade gets to set up Toxic Spikes. This is a set I think could work:
Pokemon Name: Porygon-Z
Moveset Name: Anti-Lead
Move 1: Magic Coat
Move 2: Tri Attack
Move 3: Icy Wind
Move 4: Hidden Power [Ground]
Item: Focus Sash
Ability: Download
Nature: Modest (+SpA, -Atk)
EVs: 4 HP / 252 SpA / 252 Spe
Okay, this Porygon-Z is designed to take down the majority of the top ten leads, barring Infernape and Jirachi(unless they are a bulky, slow variant).
The moves are pretty simple. Magic Coat reflects sleep moves back at Roserade and Smeargle, allowing you to come out on top unless they call your bluff, but you can still win if they do with Icy Wind + Tri Attack. Tri Attack(STAB) is the powerful attack here, and with a Download boost(easily received from Metagross, Swampert, or Hippowdon) is extremely powerful. Icy Wind is like the Bullet Punch of lead Metagross, or Extremespeed of Dragonite, only operating under different circumstances. You slow down Aerodactyl and Azelf enough to be finished off the next turn, without taking a hit! Finally, there is Hidden Power [Ground], which allows you to deal a ton of damage to Metagross, and if you get the chance to, Jirachi and Infernape.