As part of the story, you will ride a boat into a town called "Assac". In order to make progress through Assac, you will find an area called "The Arena". The eastern most part of The Arena has an arena, where there's a lion and his handler. Talk to the handler to start the fight.
This fight isn't that hard, but if you do it right when you get to Assac it will take a while. My party's levels were all around 18-20, and I had bought most of the best gear available in Assac before the fight.
The way I won the fight, was every turn Alina was healing, Hoston and Kruller were attacking, Bounty and Edessot were gathering gather and then sending it to one of the other three to keep them at max (Alina doesn't need to have max, 2 or 3 is good enough). Not sure how much health the lion has, but 15-20 turns later it was defeated. You also get 1000 golds for winning.
Also, Ryan the Lion's element is fire, so equipping Dragon Teeth to your characters will reduce the damage in half. I only had one equipped to Kruller, since he's by far the character with the least HP. Not sure if there's anywhere you can buy them in Assac.