Would a Porygon-Z work better with Agility or Nasty Plot? I've been advised to use both before, but that seems...off, for some reason.
(EDIT IF IT HELPS)
My current set is
Porygon-Z @ Wise Glasses
Modest
Adaptability
42 HP / 252 Satk / 28 SpD / 188 Spe
-Tri Attack
-Dark Pulse
-Ice Beam
-Agility
Agility Porygon-Z works best if you don't bother maxing it's speed... then you can put a little bit of it into HP or defenses. 263 speed should be enough (188 EVs) so that when it doubles, you outrun Timid Scarfgar. 253 speed (148 EVs) to outrun Timid Deoxys-S, 230 speed (56 EVs) outruns Scarfchomp and Scarf-Z.
Obviously, the goal is to remove Blissey first and then rip stuff up, but Tyranitar may cause problems, so I would remove Dark Pulse for HP Fighting, and add 4 speed EVs to make up for the lesser IVs.
If you want to use Nasty Plot, I always recommend it in tandem with Substitute. I'll have to look, but I made an EV spread that had a substitute that could survive Cress's Ice Beams all the time, and it could survive Blissey's occasionally, allowing you to Nasty Plot in it's face. You Nasty Plot, they switch-in Bliss, you Sub as they thunderwave, you Nasty Plot again as they break the sub. Then the mind-games begin. Depending on your opponent, you either Tri Attack (2HKO), or Sub again. If you are smart, you will end up killing Blissey and still have a Substitute. You'll probably want to use Salac Berry as an item, because a +4 +1 Porygon-Z WILL sweep their team, however, I would practice with a Lum Berry first until you get a feel of the set.
The EVs for this set are 8 HP / 210 Satk / 40 SpD / 252 Spe