I used Zanmato for Penance, Nemesis and every Dark Aeon except Dark Yojimbo. You've got to fight Dark Yojimbo five times consecutively, so Yojimbo's only useful once or twice - once on his own Overdrive, then once when Grand Summoned; I didn't have enough gil to be sure of getting Zanmato the rest of the time. So I just beat him the old-fashioned way, using Hastega and Quick Hit, then sacrificing an Aeon every time Dark Yojimbo used his own Zanmato, which utterly annihilates everyone on-screen - even if they're using Auto-Life.

Stat-building takes so much time; Zanmato is certainly 'cheap', but it's no less a victory.