JoycapJoshST How about actually doing calcs with a boosting item since that's what Marshmellow-kun's gonna hold the majority of the time.
252 Atk Life Orb Marshadow Close Combat vs. 252 HP / 252+ Def Skarmory: 142-168 (42.5 - 50.2%) -- guaranteed 3HKO after Leftovers recovery
252 Atk Life Orb Marshadow Close Combat vs. 252 HP / 4 Def Skarmory: 185-218 (55.3 - 65.2%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Life Orb Marshadow Spectral vs. 248 HP / 96+ Def Mega Venusaur: 133-156 (36.6 - 42.9%) -- guaranteed 3HKO
252 Atk Life Orb Marshadow Close Combat vs. 252 HP / 144+ Def Hippowdon: 175-207 (41.6 - 49.2%) -- guaranteed 3HKO after Leftovers recovery
WITHOUT needing a boosting move, Marshadow already does heaps of damage to physical walls that they can't switch in safely. And this is JUST with Life Orb and not accounting for extra Spikes or Stealth Rock damage; if it's holding a Choice Band (ala Terrakion), there's no way any frailer's gonna take more than a hit or two. Granted, you might have found a slight solution in Mega Venusuar, but that's not used nearly enough on most team archetypes outside of balance (stall prefers other megas like Sableye, offense prefers something faster like Lopunny or Charizard).
Yes, it's revenge killable especially by Scarf Keldeo and Mega Lops, but neither of them can switch in safely and unlike Terrakion its typing saves it from being sniped at -1 by random Ground-, Steel-, and Water-type attacks common on other revenge killers. Like Terrakion, Marshadow doesn't actually give many craps about coverage because CC annihilates most switch-ins while Spectral Thief is generally powerful enough. Unless you have huge scizor phobia screw fire coverage since Close Combat's hitting most neutral steels harder.
Overall it's almost like Mega Lucario in that its typing isn't easily exploitable, has accurate STABs which offer good coverage, doesn't need setup to blow teams off the earth, and its Speed and natural power is good enough to overwhelm most teams even with its share of revenge killers. If that doesn't at least ring a few warning bells I don't know what will.