Taupy Tauplin (Sands of Destruction) vs Sanctuary Keeper (Final Fantasy X)
Sanctuary Keeper has badass magic damage, which is a good start for how to kill Taupy. And it is indeed enough to kill Taupy for 95% of the game, but unfortunately for him Taupy's ultimate weapon from his sidequest has +50% max HP, so Keeper doesn't quite make it to a OHKO, and Taupy can heal it off easily enough while still beating down. That leaves some kind of status blitz, but Taupy can null Sleep & Confusion. Darkness is pretty annoying, actually, tanks Taupy's damage, and Silence can turn off the healing spigot, but will still require 2 more damage turns to finish the job. Seems like too much has to go right for the Keeper to win this.
Baal (Diablo 2: Lords of Destruction) vs
Terra Branford (Final Fantasy VI)The good news for Terra is that the vast majority of Baal's stuff is Fire / Ice / Lightning and thus totally nulled, and his physicals ram into her evade, so a Drain strategy seems promising. The bad news is that Baal has something like 30 PCHP (even if you scale solo bosses against a party of 3 or 4 normally, Baal's HP multiplies by the number of party members!) and an MP halve move. So Baal can bust Terra's MP out, then beatdown once her Drain isn't a factor anymore. The good news is that D2 bosses don't heal when you leave them (unlike Diablo 3), and yes, Terra learns Warp as a spell straight-up. So Terra just warps out of battle once her MP gets too low, runs back to the airship, heals, then runs back to non-healing Baal. You can do it to Doom Gaze, and Terra really does have Warp as a legit spell, and Baal doesn't have a can't-town-portal-flag (again unlike D3 bosses), so this seems a clean if unusual victory via WARP hype.
(For those curious since there's no stat topic,
http://www.gamefaqs.com/pc/370600-diablo-ii-lord-of-destruction/faqs/18357has a decent rundown on Baal's options. Search for "Incineration Nova".)
Light
Amalthea (Legend of Heroes: Trails in the Sky) vs Kongol (Legend of Dragoon)
Amalthea durability is not great but it's not nearly bad enough as to eat a Kongol OHKO, which is what he needs.