ive had success with P2 in ou against dragonite and to a lesser extent salamence and haxourus. also gyarados and gliscor, but is used primarily for dragons
my set:
Bold nature @eviolite (duh)
252 defence, 252 HP, 4 SpDef
trace ability
Icebeam
Thunderbolt
thunderwave
recover
anyway with this i switch in to Dragonites, ususally while they set up a dragon dance. trace the multiscale, at this point im guarenteed to survive any attack it deals. thunderwave it, so just incase it deals over half, i can recover if neccisary. often they will bust out an outrage, so the parafusion comes in handy. at this point i can recover up to full health AND icebeam it to death, with multiscale broken, depending on its build. plus at that point, being at full health with a eviolite and multiscale, nothing can OHKO it (well probably, never have been so far and ive been using this set for months) let alone 2KO it.
additionaly, no one seems to expect porygon2 having both thunderbolt and icebeam for what ever reason. and often found people who carry a dragon or gliscor with gyarados and never suspect i carry both so the switch in on porygon to absorb thunderbolts (dragons and gliscor) or icebeam (gyrados) only to be surprised. what ever.
also i love the trace ability. a few times actualy ive been toxic'd, then switched in on gliscor, traced poisen heal and pretty much become a superwall.
also the only checks ive found so far are reuniclus and gastrodon...
i often run this on my sun team, and is essentially the backbone. i believe it should be in ou, but alas.
just thought i'd share this