On second thought, Porygon2 gets Agility. A set with, say, Agility/Tri-Attack/Thunderbolt/HP Fighting with a Life Orb may very well be too broken for UU. I didn't think of that at first.
Assuming it pulls off a Download boost, nothing avoids being 2HKOed.
Modest Porygon2 @Life Orb Damage Calculations (assumes Download boost, which is not too unlikely):
Tri-Attack on 252/252 Calm Blastoise: Damage: 46.96% - 55.25%
Thunderbolt on 252/252 Calm Blastoise: Damage: 74.31% - 87.29%
Tri-Attack on 252/252 Calm Lanturn: Damage: 46.04% - 53.96%
Tri-Attack on 252/252 Careful Muk: Damage: 42.51% - 50.00%
Psychic on 252/252 Careful Muk: Damage: 63.53% - 74.88%
Tri-Attack on 252/252 Calm Meganium: Damage: 48.35% - 56.87%
Ice Beam on 252/252 Calm Meganium: Damage: 76.37% - 89.84%
HP Fighting on 252/252 Calm Probopass: Damage: 95.99% - 112.65%
Even if you don't manage the OHKO, Probopass can't do much back
HP Fighting on 252/252 Careful Bastiodon: Damage: 101.85% - 119.75%
Best it can hope for is Focus Sash + Metal Burst (as if anyone would use Bastiodon)
HP Fighting on 252/252 Careful Steelix: Damage: 75.71% - 88.98%
(Like anyone is going to use 252/252 Careful Steelix)
Thunderbolt on 252/252 Calm Lapras: Damage: 61.85% - 72.84%
This would leave Lapras with almost no offense, however, and there's always Stealth Rock.
Thunderbolt on 252/252 Calm Noctowl: Damage: 70.54% - 82.92%
OHKO with Stealth Rock
Thunderbolt on 252/252 Careful Poliwrath: Damage: 77.34% - 91.15%
No Poliwrath is this defensive, and if it is, it can't OHKO back.
Tri-Attack on 252/252 Calm Altaria: Damage: 48.02% - 56.50%
Potential 2HKO, always a 2HKO with Stealth Rock. Again, this is the most defensive Altaria possible, and if it's using this, it certainly can't do much back.
All the common Fighting-types are at least potentially OHKOed by Tri-Attack unless they invest massively in Sp. Def.
So basically, the only things I can see reliably stopping this are users of STAB Mach Punch, none of which can switch in. Even then, Hitmonchan's Mach Punch does not OHKO even with a Choice Band. Hitmontop's does even less.
It also has access to Psychic and Ice Beam if you need them, mainly for Muk/Nidoqueen/Nidoking or Meganium/Nidoking/Nidoqueen, respectively.
Obviously, I've changed my opinion. If Porygon-2 manages a Download boost, nothing can switch in without a serious risk of being 2HKOed. Belongs in BL.
Instead of bringing down numerous walls to counter what's already in UU, why don't we work on trying to establish a consensus on what threats that already exist need to be removed.