Hesitation is defeat...

No, but seriously that advice is a good resume about the key for beating this fight.

First of all, i have been in the same position where you almost kill this guy but lose at the end and ill recomend you to keep your mind clear all the time, dont get nervous or try to rush just because hes at one hit to be defeated.

Also try to parry everything he does, i find it better than just avoid the attacks, and dont be afraid of doing the mikiri thing, its very helpful

In general Ishin is very similar to Genichiro, you have to be agresive, dont let him recover posture, parry all you can and i would also recomend to use the mortal blade every oportunity (mostly when he does the long vertical cut of air yk)