If you use Blissey correctly, I am pretty sure that it would be able to take him down, even if Blissey couldn't, Yonowaaru still could, hell I could use T Tar to get rid of it, yes the Pokemon I have stated are Over Used all the time but I don't care seeing as how I like to win. So the way I see it anything strong enough to win is fun to me and any of those could go toe to toe against Porygon Z.
Don't get me wrong though, I know Porygon Z is very good, I plan to have one on my Team myself as Porygon was one of my first Favorites.