Zanmato? Why do you want to do that for? I've beaten Penance using Zanmato before, but since its against my values to cheat like that on the most difficult boss in the game I decided to beat Penance in overkill
Actually, to tell you the truth, since I don't think you get anything cool from beating it I never bothered after losing two times. I will do it someday though.
ANYWAY, Try getting all your stats up to 255, except for magic, it's not needed at any point in the game (not hard, and not long if you know which stats to boost first, and have the right weapons) and apparently you need 160 luck.
When you engage with Penance attack his arms with quick hits and attack his main body with Attack Reels (yes, you do have to be able to hit 12 times each attack reels).
have your overdrive set to comrade and use Yuna, Lulu, and Wakka and their celestial weapons. (Yuna and Lulu because of their one mp cost means semi-unlimited auto-life casting and semi-unlimited quick hits) (Wakka is your heavy artilery, since he has the most powerful attack in the game)
if you want to use Zanmato you can. If Yojimbo doesn't use it then it probably had something to do with your choice when you first met him
The I is moving 
I would rather live in my own shadow...at least then I would be compared to a past I'm more familiar with ~I~
Fear me, Hate me, Curse me, for I will question your every move ~I~
My past hold much pain and loss. I will conquer it by creating the perfect future ~Ixidor the Reality Sculptor (Magic the Gathering)~
