Wouldn't Ice Punch just be more useful for not forcing Pheromosa to use her uninvested SpA? And if Thunderpunch gets past Toxapex and Mantine without sacrificing coverage for Fini or Azumarill, seems useful How does Scizor fare against a theoretical Fire Punch with Rocks in play?
Regardless, theorymonning aside, what are the good Poison types in OU and how would they match up against Pheromosa? That seems to be the type that fares the best against Pheormosa of its STAB resistors, compared to Flying dealing with Ice Beam and Poison Jab dealing with Fairies. Ghost fares about as well in theory, but that typing basically has Gengar, Sableye, and Mimikyu in OU right now to my knowledge, less variance than Poison types I'd imagine, even if only by a small margin.
I think Ice Beam would be the choice move given that the relevant threats hit by Ice coverage are actually hit harder the uninvested Ice Beam, primarily because said defensive sets are geared towards physical -- if you run through the sets on the Damage Calc.
4 SpA Life Orb Pheromosa Ice Beam vs. 248 HP / 8 SpD Landorus-Therian: 530-624 (139.1 - 163.7%) -- guaranteed OHKO
-1 252 Atk Life Orb Pheromosa Ice Punch vs. 248 HP / 244+ Def Landorus-Therian: 234-276 (61.4 - 72.4%) -- guaranteed 2HKO
4 SpA Life Orb Pheromosa Ice Beam vs. 244 HP / 48 SpD Tangrowth: 354-416 (88 - 103.4%) -- 25% chance to OHKO
252 Atk Life Orb Pheromosa Ice Punch vs. 244 HP / 216+ Def Tangrowth: 140-166 (34.8 - 41.2%) -- guaranteed 3HKO
4 SpA Life Orb Pheromosa Ice Beam vs. 252 HP / 0 SpD Zapdos: 242-286 (63 - 74.4%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Pheromosa Ice Punch vs. 252 HP / 172+ Def Zapdos: 192-229 (50 - 59.6%) -- 80.1% chance to 2HKO after Leftovers recovery
4 SpA Life Orb Pheromosa Ice Beam vs. 248 HP / 92 SpD Amoonguss: 239-283 (55.4 - 65.6%) -- guaranteed 2HKO after Black Sludge recovery
252 Atk Life Orb Pheromosa Ice Punch vs. 248 HP / 168+ Def Amoonguss: 221-260 (51.2 - 60.3%) -- 91.8% chance to 2HKO after Black Sludge recovery
4 SpA Life Orb Pheromosa Ice Beam vs. 240 HP / 0 SpD Garchomp: 510-603 (122.3 - 144.6%) -- guaranteed OHKO
252 Atk Life Orb Pheromosa Ice Punch vs. 240 HP / 176+ Def Garchomp: 354-421 (84.8 - 100.9%) -- 6.3% chance to OHKO
4 SpA Life Orb Pheromosa Ice Beam vs. 192 HP / 64 SpD Pelipper: 136-161 (44 - 52.1%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Life Orb Pheromosa Ice Punch vs. 192 HP / 0+ Def Pelipper: 101-120 (32.6 - 38.8%) -- 5.3% chance to 3HKO after Leftovers recovery
In addition, Ice Beam would bypass any rocky helmet damage.
As for fire punch to answer your question...
252 Atk Life Orb Pheromosa Fire Punch vs. 248 HP / 60+ Def Scizor: 380-452 (110.7 - 131.7%) -- guaranteed OHKO