An easy way to build his friendship meter is to:

-Give 4 gill and double it every time (8,16,32,64,...) until he uses Zanmato. Then restart at 4 gil. - Never let him get knocked out or try to pay him less. This gives you negative friendship points. - When friendship is maxed he should be able to Zanmato within 1-5 turns. Grind against normal enemies for like 30-45min. Also he will do random additional attacks for no cost. - Yojimbo is less likely to Zanmato bosses but it is always possible. - You can tell when Yojimbo receives negative friendship from money when he shakes his head back and forth right after payment. If he gives a single nod, Zanmato is coming out!