If for whatever reason you reallllly need to go COR/NIN with a multihit weapon and melee, the best thing to put in your sub slot is a Genin Aspis. Yes, it's uncommon.
/NIN to me pretty much means pulling only, so I don't usually care what's in my offhand. I usually equip my Auster's or Light staff when I pull on /NIN.
And BTW, COR/DNC to me is far more effective in Nyzul than /NIN. Instant cast Sneak+Invisible with Spectral Jig (free too) is better than Tonko/Monomi. You just need to be slightly more careful with taking AoE, but the extra cures and status removal from Waltzes are really helpful. Most importantly, stacking Box Step on those stronger mobs (NMs, bosses, tough families) is a life saver. I don't bother coming as anything but COR/DNC any more to Nyzul.
Anza: Titan 2004-2011 / Capuchin: Phoenix 2011-???