I think Aoshi 1 is (technically) the only fight Kenshin has where he wouldn't have killed his opponent with a proper sword
This just an observation that I found interesting. The majority of the time the choice of weapon makes the difference, and so many times Kenshin would have killed with a proper sword, except for 1 time.
Various henchmen, Gohei, Sano, Hannya, Raijuta. Saito, Senkaku, Sojiro, Aoshi 2, Shishio, Enishi... against almost every single opponent Kenshin has landed a blow that would have crippled or killed an opponent. Aoshi is basically the only opponent he never actually hit with his sword. He used the hilt of his own and Aoshi's own sword to hit Aoshi in the throat two separate times, knocking him out.
The reason I say technically is just because it's demonstrated that past Kenshin wasn't above finishing a downed opponent to make sure they're dead, so unless the other Oniwaban got there faster he would still kill Aoshi.
I'm uncertain how to classify Gein, though, as he might be the second. However, if Kenshin wrecked his suit with ARNH that badly with the dull side of a sword I can only assume Gein would have died if Kenshin used a proper sword with the move as it would be much faster and thus stronger.
Gein even says he would have been finished without the impact absorption mechanism and is too drained to keep fighting afterwards, so a little more power, even without direct contact, feels like it should be enough to kill him.
I guess if you REALLY want to stretch the definition of a "fight" the artist guy with the bombs would live, but the fight got interrupted before Kenshin would need to land a blow and it's obvious he would have won if it had continued.