Ok i´ll give you the easy way out of it...not the most honourable anyway...

Ok so first it´s important that you have been using Yojimbo a lot so he becomes more attached to Yuna.
Then make sure that both Yuna and Yojimbo have their Overdrives ready. This means that you will be able to perform two Zanmato atacks on the two first fights...if of course you have used him properly.

Then for the other fight you will have to wait for lady luck to strike and that Yojimbo uses Zanmato once again...

I used this technique once and it worked!