It has long been established that Zanmato kills any and all monsters if Yojimbo chooses to perform it. HP shouldn't even be a problem, since Zanmato doesn't even do damage. Before Int. came out, this was the preferred method of killing Nemesis.

100k? Try 1 million gil, International, or not. Overdrive will not affect your chances of getting Zanmato, since Zanmato affects all enemies on screen, regardless of number of enemies, and it is not an Overdrive, it is considered a Special attack, just as Impulse is Bahamut's Special attack, and Mega Flare is an Overdrive.

So I shall test the method without turning on my PS2, giving him 100,000 gil. My success rate? Assuming I have not used Zanmato before, my success rate is 0%. If I have use Zanmato before, it means I have already surpassed 100,000 gil, already paid Yojimbo more than 1 million in funds, and used it to raise my companionship rate with Yojimbo, which means it will be at any number above 10%.

Of course, International is for the very weak minded and those who lack strength of thumb, so I could buy that 100k gil will get you Zanmato...

For those who have not yet gotten Yojimbo, please consider everything you read here a spoiler.