I'll edit this post later with a few calcs.
It's actual calc time!
https://pokesports.org/pages/damage-calculator
EDIT: Ffs, apparently the various abilities do not work. I tried to see the calcs outside the sun and they didn't change. The Ruin abilities' calcs didn't change when I changed the ability to something else.
This is annoying :(
Vs Garganacl [Shaky check]
252 SpA Choice Specs Flutter Mane Moonblast vs. 252 HP / 252+ SpD Garganacl: 186-219 (46 - 54.2%) -- 4.7% chance to 2HKO after Leftovers recovery
252 SpA Choice Specs Fairy Tera Type Flutter Mane Moonblast vs. 252 HP / 252+ SpD Garganacl: 248-292 (61.3 - 72.2%) -- guaranteed 2HKO
252 SpA Choice Specs Ghost Tera Type Flutter Mane Shadow Ball vs. 252 HP / 252+ SpD Purifying Salt Garganacl: 104-124 (25.7 - 30.6%) -- guaranteed 4HKO
4 Atk Garganacl Heavy Slam (120 BP) vs. 0 HP / 0 Def Flutter Mane: 280-330 (111.5 - 131.4%) -- guaranteed OHKO
4 Atk Garganacl Heavy Slam (120 BP) vs. 0 HP / 0 Def Ghost Tera Type Flutter Mane: 140-165 (55.7 - 65.7%) -- guaranteed 2HKO
Vs Ting-Lu [Checks once] EDIT: Ok, apparently the Ruin ability doesn't appear to work properly in the calculator! So... this is useless. I'll go run face first with Ting-Lu against FM and I'll see the results.
252 SpA Choice Specs Flutter Mane Moonblast vs. 252 HP / 252+ SpD Ting-Lu: 398-470 (77.4 - 91.4%) -- guaranteed 2HKO
252 SpA Choice Specs Fairy Tera Type Flutter Mane Moonblast vs. 252 HP / 252+ SpD Ting-Lu: 532-628 (103.5 - 122.1%) -- guaranteed OHKO
4 Atk Ting-Lu Throat Chop vs. 0 HP / 0 Def Flutter Mane: 153-180 (60.9 - 71.7%) -- guaranteed 2HKO
4 Atk Ting-Lu Heavy Slam (120 BP) vs. 0 HP / 0 Def Flutter Mane: 304-358 (121.1 - 142.6%) -- guaranteed OHKO
Vs Chi-Yu [Checks once]
252 SpA Choice Specs Flutter Mane Moonblast vs. 252 HP / 252+ SpD Chi-Yu: 153-180 (48.7 - 57.3%) -- 92.6% chance to 2HKO
252 SpA Choice Specs Fairy Tera Type Flutter Mane Moonblast vs. 0 HP / 116 SpD Chi-Yu: 248-292 (98.8 - 116.3%) -- 87.5% chance to OHKO
252 SpA Chi-Yu Fire Blast vs. 0 HP / 4 SpD Fire Tera Type Flutter Mane in Sun: 107-126 (42.6 - 50.1%) -- 1.2% chance to 2HKO
252 SpA Chi-Yu Fire Blast vs. 0 HP / 4 SpD Ghost Tera Type Flutter Mane in Sun: 214-253 (85.2 - 100.7%) -- 6.3% chance to OHKO
This is Scarfed Chi-Yu
Vs Tyranitar [Meh, but better than others]
252 SpA Choice Specs Flutter Mane Moonblast vs. 252 HP / 252+ SpD Tyranitar in Sand: 230-272 (56.9 - 67.3%) -- guaranteed 2HKO
252 SpA Choice Specs Fairy Tera Type Flutter Mane Moonblast vs. 252 HP / 252+ SpD Tyranitar in Sand: 308-364 (76.2 - 90%) -- guaranteed 2HKO
4 Atk Tyranitar Crunch vs. 0 HP / 0 Def Flutter Mane: 180-213 (71.7 - 84.8%) -- guaranteed 2HKO after sandstorm damage
4 Atk Tyranitar Stone Edge vs. 0 HP / 0 Def Flutter Mane: 225-265 (89.6 - 105.5%) -- 37.5% chance to OHKO
252 SpA Choice Specs Flutter Mane Moonblast vs. 252 HP / 252+ SpD Assault Vest Tyranitar in Sand: 152-182 (37.6 - 45%) -- guaranteed 3HKO
252 SpA Choice Specs Flutter Mane Moonblast vs. 252 HP / 140+ SpD Assault Vest Tyranitar in Sand: 168-200 (41.5 - 49.5%) -- guaranteed 3HKO
252 SpA Choice Specs Fairy Tera Type Flutter Mane Moonblast vs. 252 HP / 140+ SpD Assault Vest Tyranitar in Sand: 224-268 (55.4 - 66.3%) -- guaranteed 2HKO
112 Atk Tyranitar Stone Edge vs. 0 HP / 0 Def Fairy Tera Type Flutter Mane: 246-289 (98 - 115.1%) -- 81.3% chance to OHKO
112 Atk Tyranitar Crunch vs. 0 HP / 0 Def Flutter Mane: 195-231 (77.6 - 92%) -- guaranteed 2HKO after sandstorm damage
A couple notes about this scenario:
1) The opponent's team has Torkoal. So it's 6 vs 5,5.
2) Torkoal does not have a pivot move! FM has to come in through other means. Double switch, pivots from other mons.
3) Prothosyntesis needs Sun to work, so in a pinch another weather could help (but this just means the meta is more weather and I hate that.) This also means that FM does not have 5+ free turns to spam its STABs. (Just 3, lmao)
That said, a good opponent will be able to pivot around and have a couple turns throughout the match when FM is against something, with sun up.
This is NOT the only set FM can have, just the one that hits harder and makes sense to use (Specs with PS giving +1 Spatk is not something we're going to see, I guess. A scarf version of it it's possible, but still won't hit as hard.)
Last thing: I'm tired. I'll go to sleep now. I'll run more calcs tomorrow.
I think with CM Leftovers Flutter Mane, CM Life Orb.