After some further testing, I'm starting to believe that Toxic might be the best move in the last slot assuming you are using Swampert with a Politoed. Superpower and Low Kick might seem useful for Ferrothorn, however they arent always guaranteed 2hkos on it when you account for leftovers and protect, superpowers drops in power, and the iron barbs recoil is really annoying too. Honestly, if i wanted to beat ferrothorn i would just use substitute since most run a setof stealth rock, leech seed, gyro ball, and protect. I feel like Power-Up-Punch doesnt work too often and most teams i sweep wih it are swampert weak anyways, meaning it rarely makes a difference. Hydro Pump is good for Skarmory, however, i usually take care of skarmory with magnezone which also takes care of Ferrothorn. Toxic takes care of the most common answers to Swampert outside of those Magnezone beats, namely Rotom-W, Slowbro, Mega Slowbro,
Celebi, Gastrodon, Tangrowth, and really any annoying bulky pokemon.
http://replay.pokemonshowdown.com/orasou-182598997
Here is a replay of Toxic working successfully. This replay is kinda undermined by my opponent choking at the end, but at the time I toxiced him he was playing well.