He's not just a Lesios, he's a Megalesios. Anyway, Megalesios is the Telkine that you've heard so much about during your travels through Greece. When you approach him, he'll create a barrier around himself, and he'll summon four inhabited statues to attack you. The statues are slow, so you can use hit-and-run tactics to take them down.
After destroying the statues, the barrier will fall, allowing you to attack Megalesios.
Megalesios casts a variety lightning spells, and he summons limos ("life eater") demons to help him out. The limos demons can drain your health and give Megalesios more hit points, replenishing his health quickly, so target them as soon as you see them.
Otherwise, just quaff potions and whack away at Megalesios until he's dead.
Strategy
Important resistances:
Lightning
Vitality
As soon as Megalesios summons the Limos, kill them instantly to prevent them filling up the Telkine's life. Also improve Vitality resistance to prevent them alfrom leeching too much life.
With 60-70% lightning resistances, Megalesios is very easy to defeat.