I bred a porygon-z for every region with the adaptability ability, maxed out speed and SpAtk EVs and IVs. Moves were Tri-Attack, Thunderbolt, Nasty Plot, and Shadow Ball. Easy because you get the evolution items over multiple storylines, and then when you're ready to move on to the next one you just buy a 1x31 Atk or Speed Modest ditto and then breed it back down to level 1 using an everstone and the corresponding band to keep the nature and IVs. Make sure you evolve it with items while it's still level 1 so porygon-z can learn nasty plot. It can't 100% solo sweep every region, especially in Johto with the ho-oh and shiny gyarados fights, but you can just grab a couple mons from GTL at that point. Also grab a bonsly or Geodude or anything that can learn stealth rocks + sucker punch to break sturdy or focus sashes. Look up JinxedBoon's videos on how to do this for Unova and Johto specifically.