Damage limit?

[Nominus Experse]

The best way to score more than 9999 is to use a weapon that has a high hit ratio (Katanas, Ninja Blades, or Poles) in conjunction with the Gengi Gloves. Five consecutive hits of 4000 means much more than one hit of 9999. And more hits means more hits that might land a Critical.

Another tactic is to cast Reflect on your entire party, and then target yourselves with Scathe.

If all things fall into place perfectly, you are still capable of hitting for 99990 damage...