There is a move that has been forgotten, you're gonna laugh, but from my experience, it has been proven very very good, Power-Up Punch, yes with this move, you get Steel Types, which resist both of your STABs, and most of them would set up so you'd exhaust Sucker Punch's PP, but Power Up Punch takes care of that.
252+ Atk Huge Power Mega Mawile Power-Up Punch vs. 248 HP / 8 Def Ferrothorn: 130-154 (37 - 43.8%) -- 99.8% chance to 3HKO after Leftovers recovery
+1 252+ Atk Huge Power Mega Mawile Power-Up Punch vs. 248 HP / 8 Def Ferrothorn: 194-230 (55.2 - 65.5%) -- guaranteed 2HKO after Leftovers recovery
so with luck, you 2HKO Specailly defensive Ferrothorn, if not then at least 3HKO, which is something you could never do with standard moveset, well unless you run Fire Fang
But it helps me so much, always playing a mind game with pokemons that try to set up fearing Sucker Punch while I use PuP, then next turn I sucker punch, usually they attack, and so they die.