ReCast FF3: War of the Magitek

Relm learns Onyx at level 82.  Mog learns it at level 75 (then Pearl, and then Garnet, but he doesn't learn the top spells).  No other characters learn it on their own.

I'm almost inclined to turn it the other way around... make more bosses resistant to poison so that percentage attacks do 0, and have these spells be designed for random monsters.
That at least staunches the problem for the most part... though Comet or Debilitator can potentially make a boss vulnerable to poison, you have up to 8 turns wasted trying to get that cheese, and you have to be deliberately going for it at that.

I'll look through the boss flags and consider adding poison resistance to more bosses (more of them need to be immune to the poison status too, honestly).  I may also reduce Onyx's damage to 1/2 and have it apply the poison status.

EDIT: Hmm, most of the World of Ruin bosses are already neutralizing poison damage. The dream stooges don't (because you're supposed to slice them up with Cyan), the dragons don't (which is admittedly an oversight on my part), and Corpus doesn't (because he thought status magic was lame and Spectre's going to teach him a lesson). Onyx might not be as big of a problem as it currently appears... did you use it to shoot through Tentacles or Chardanook?
Any which way, I'll look into setting poison immunity and poison element immunity for the 8 dragons.
...Well, maybe not all of them. Having Onyx to use on Yloh is one of the strats to beat that monster, and will be nice if you run into it repeatedly on the Veldt. Daednu gets healed by poison already. Imanust has the whole rippler thing going on, so inflicting poison status isn't as much of a cheese there.