Cecelia Lynn Adelhyde (Wild ARMs) vs Royce (Lunar: Silver Star Story)
Heavy
Therion (Octopath Traveler) vs Estelle Bright (Trails in the Sky)
I'll buy Elf's maths for Estelle's usual strategy. (I could run it for Trails SC as well, but Estelle is in general slightly worse there than FC, so I doubt it would change the outcome.) That said... this might be a rare time where Estelle drops a "turn 0" S-Craft and actually eats the awful turnspeed penalty to avoid getting Shackled, then hopes a late-turn-2 Blue Impact seals the deal. For this to be effective, she'd need to gain more from the extra S-Craft damage then she loses from giving up a Blue Impact. Maybe?
Vyse Inglebard (Valkyria Chronicles) vs Camilla (Fire Emblem Fates)
Hmm, do people see Attack Down as debuffing both "physical" attacks and "magical" attacks? Probably not. Even if it's only physical, Camilla does way, way less damage with a Bolt Axe. On the bright side, Savage Blow is great for plowing through a lot of HP. Hmm.. seems ATtack-Down Vyse 5HKOs Camilla (but counters). If we let Camilla go first due to her high threat range, she opens with a Silver Axe (32 dmg) + Savage Blow, eats Vyse counter, Vyse attacks (no counter), Camilla Bolt Axe (13 dmg) +SB, Vyse counter, Vyse attacks, Camilla gets one last Bolt Axe attack (13 dmg) then Vyse finishes his 5HKO on the counter (or even 6HKO if needed since he'll have his own turn). So can 58 Fates damage + 2 Savage Blows beat up Vyse? Sure, with a killpoint of 55 Fates damage, that's ~1.4 PCHP vs. average, but Vyse has above-average HP so the Savage Blows do even more, and Vyse has 1.44 durability.
Thage (Eternal Poison) vs Jade (Dragon Quest XI)
No EP... but... assuming Maijin's aren't relevant and Thage is forced to pick Hellcaster or Flameweaver form first, Jade can equip against either Fire or Dark as appropriate, then probably go first & 2HKO Thage's bad HP while not getting OHKO'd is my kneejerk based off the stat topic.
Middle
Emma Millstein (Trails of Cold Steel) vs Albel Nox (Star Ocean: Till the End of Time)
Yeah, SO3 damage averages are hard to peg, but unless Albel truly fails at damage, Emma needs to set up something crazy like a full set of Speed buffs, some ATS buffs, then double turn for 2 spells -> S-Craft. Only Light physical duelists should ever let this happen.
Tressa Colzione (Octopath Traveler) vs Squall Leonhart (Final Fantasy VIII)
Well, Squall could attack himself, and then multi-hit Renzokuken through Sidestep spam... but I'm pretty sure if he's that low, a Tressa turn 2 boosted Tradewinds, pathetic damage though it is, finishes him off. If it's not a secure kill, equip Elemental Heavy Armor or something.
Light
Python (Fire Emblem Echoes: Shadows of Valentia) vs Jeanne (Jeanne D’Arc)
Python's suspect damage bites him here, as long as Jeanne doesn't get antsy and ram herself into too many counters. Python can't go to Steel Bow or else he'll start getting doubled (Iron Bow Python is exactly average), so he 5HKOs or thereabouts. Jeanne launches a single attack, eats more attacks while busily doing nothing, eventually transforms and hopes to finish the job with a Visiting Goddess which should easily finish the job along with the original attack. She can even still win with a miss.
Alanis (Suikoden III) vs Orochi (Fire Emblem Fates)