Yeah, I'm not sure if this is a bug or just easily explotible: But if you land the slow effect of Ramei twice, the final boss's timer stops and he will not attack until the effects wear off... so spam Ramei and you win. Use poison from Hell Thorns to speed up the process if you feel like it.
"Doing the work of a small army." Seriously, I just killed a dozen shirtless, unarmed guys... and you have armor and an axe. How did you lose this keep again?