Is Charge Beam viable on Trick Room Porygon2 over Thunderbolt, i.e. Charge Beam/Ice Beam/Recover/Trick Room? It can 2HKO Gyarados, while recovering the damage from Waterfall (which can only 3HKO if Gyarados gets lucky, taking Leftovers into account).
I would switch into Gyarados and take another attack (or let it switch out), recovering off the damage. Then if it stayed in, I'd Charge Beam once, Trick Room, recover off damage again, and finish Gyarados with another Charge Beam.
Plus, it has the benefit of a 70% chance to increase Sp. Atk (63% taking into account 90% accuracy). I often find that Porygon2's Sp. Atk isn't quite high enough to deal with more defensive Pokémon, so Charge Beam seems quite useful.
Is there anything threatening that Thunderbolt would KO that Charge Beam can't?