Onimusha: Warlords

Just a little tip if you are trying to do this without upgrading weapons or healing or using jewels, you are going to want to get the bishomon sword out of dark realm. That's a huge pain but at least you'll be able to mop the floor with this guy without having to watch the unskippable cutscene over and over.

Now, you should be aware that it's still really hard if you dont use the bishomon sword correctly. You want to use the magic attack with "y" when he slumps forward ONLY....he can take 8-9 swings to stagger with the magic attack. Even though you have unlimited magic and he looks like he is getting hurt more with the blue fire all over his body, you want to use regular attacks with "x" to make him slump over first. THEN smack him in the head with "y". He'll go down much faster this way, which is good, because your tiny health bar can only take about 3 hits in this fight.

I'll post some dark realm tips on that achievement page.

TheOnlyMatto has an alternative method which may be easier for you:
'way easier to split the two and face him twice. just use medicine the first time and have a save outside the demon door. then go back, upgrade, fight to the bottom of the dark realm and complete the game again."