It's not random - the more Yojimbo likes your team, the more likely he is to do Zanmato without a lot of Gil.

Every time you summon him, pay him more than a Gil, and let him defeat the enemy, he likes you more. Every time you summon him, pay him nothing, and let him get defeated, he likes you less.

Zanmato is more likely to happen if (1) you pay him more and (2) he likes you a lot. The more he likes you, the less you have to pay him. Also, a full overdrive gauge makes Zanmato more likely.

Galadus, please don't post more than once in a row. Use the 'edit/delete' button if you make a mistake you wanna fix.