Go back into the Centra Ruins, without Enc-None on. Stick near the entrance. You will fight Tonberries. They are little ugly green dudes with knives. They are pretty high on defense, but persevere. Use Diablos and your kickass Physical attacks. My advice is, Keep Squall in critical status and repeatedly do his Renzokuken. Be sure you can all revive those killed by the little fishy assassin. Save between battles outside on the map. Defeat 20 Tonberries, and as the 20th battle comes to defeat, you will automatically fight the Tonberry King.
Kick his ass, he is not hard. Do the Renzokuken with Squall in critical or cast Aura. He isn’t really a powerful GF either, but his abilities such as: Haggle, (lowers prices), familiar (makes rare items available to buy) call shop (call and buy things from a shop from your menu screen), Sell High (get more money for sold items) and Lev Up or Down makes enemies easier to kill an worth less, or harder and worth more. It is definitely worth the trouble. Be sure to refine all those chef’s knives into AP ammo for Irvine!