Just a note, malt can run special sets or use the more powerful recharge moves (hyper beam and giga impact), so the return point is a bit misleading and it does check marsh.Tapu Koko: I'll give you this one.
Tapu Fini: I'll also give you this one.
Altaria-Mega: 252+ Atk Pixilate Altaria-Mega Return vs. +1 0 HP / 120 Def Marshadow: 270-320 (84.1 - 99.6%) -- guaranteed 2HKO
+1 136 Atk Marshadow Soul-Stealing 7-Star Strike vs. 252 HP / 4 Def Altaria-Mega: 390-460 (110.1 - 129.9%) -- guaranteed OHKO
Celesteela: 252 Atk Marshadow Close Combat vs. 252 HP / 252+ Def Celesteela: 135-160 (33.9 - 40.2%) -- 41% chance to 3HKO after Leftovers recovery
252 Atk Marshadow Soul-Stealing 7-Star Strike vs. 252 HP / 252+ Def Celesteela: 219-258 (55 - 64.8%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Celesteela Air Slash vs. 0 HP / 4 SpD Marshadow: 186-222 (57.9 - 69.1%) -- guaranteed 2HKO
In short, the only way Celesteela wins all the time is with Specs or something.
Diancie: 252 Atk Marshadow Soul-Stealing 7-Star Strike vs. 0 HP / 0 Def Diancie: 219-258 (90.8 - 107%) -- 43.8% chance to OHKO
And no, bulky regular Diancie is bad.
Landorus-Therian: Not gonna go into logistics, but Will-O Marsh won unless it was Flyinium, which is suboptimal, and was only used because Marsh was still around.
Lopunny-Mega: Loses to bulkier Marshadows.
Mawile-Mega: You got me on this one, unless you run into that shitty Roseli Berry bait set that was running around.
gonna edit in more once i have time
Subflail lop should basically always win, as if marsh tries to bulk up it can drop an HJK and even bulky marsh isnt *that* bulky.