Either get it off Tchakka or select it as a reward from killing off a set of the WKRs, then use a paragon KI to put the OAT augment on it. Ideally you may want to use either Hurkan's paragon for acc, or Achuka's paragon for atk. (Obtain paragons by beating WKRs with the relevant Records of Eminence objective active)
Basically the Atoyac (and all the other i115 weaponry the WKRs drop) are pretty solid weapons in their own right, upon which you can place a OAT augment that is nearly equivalent to the Joyeuse's rate of OAT. Remember how popular of a weapon Joyeuse was despite being a level 70ish weapon and not otherwise particularly exciting? Yeah.
On NIN nowadays, I typically use the i115 katana they recently added to Colkhab's drops, along with the Atoyac even though it's a dagger. I adjust which one goes in my mainhand based on what I'm fighting (whether I want a slashing or piercing WS, basically). Wielding two of these OAT weapons pretty much shreds through any sort of older content. I don't have to worry about doing harder content on NIN, because it's not like it'd ever get invited anyway. Sadly, as far as THF+DNC goes, the Atoyac cannot be doubled up on as it's a rare :(
Edited, Oct 2nd 2014 11:26am by Fynlar