This chapter of Assassin's Creed: Odyssey walkthrough includes the description of how to defeat the Nemean Lion in Argolis.
Suggested character level: 22
Description: The Lion is the second of the legendary beasts you encounter when traversing the map in Assassin's Creed: Odyssey. It is much bigger than a regular lion and has more health points. This beast avoids most of your attacks and performs quick charges. It is not worth shooting at it with a bow or uses ranged weapons, as the beast dodges most of them. Its quick move makes it better to stay close and deal direct damage.
Every once in a while your opponent will call 2 or 3 lions sharing the same level as the Nemean Lion. After you eliminate the uninvited guests, continue your fight with the boss. This situation will repeat itself several times. You have to be prepared for a long and monotonous battle.
You receive 8700 experience points for completing this quest. Return to Daphnae with a hide to receive the legendary Pandora's Dagger.