Leonyx must be defeated in order to access the Dark World, the final level in the game. He is twice as difficult as his Malevolynx forme. Leonyx swings both his giant swords twice and uses the ability Miracle Slash to restore health. His most deadly attack, however, is Kazap, which causes lightning damage. Having a monster like the alabast dragon with the resistance Healed By Zap at a high enough level (app. Lvl. 25) on the team can negate the effects of Kazap and actually heal it. The cumulative damage by Leonyx will eventually deplete its health, however, so potent medicines must still be on hand to use.
Another good choice is to recruit a night clubber and use its Giant Killer skill.
Leonyx will upgrade the hero's Scout Ring post-game. It will then be possible to scout the bosses (Wormonger, Missing lynx, Khalamari, etc.).