For clarity, the
Elemental Lord is the ultra-tough ice golem. Not to be confused with the other high-level monsters in that immediate area.
He's a tough boss so don't expect to take him on until the late game. Likely well after you can beat the nearby ice dragon.
Basic boss fighting tactics will serve you well.
Equipment: He hits hard with physical and ice attacks so gear up that way. End game gear (Sunken Citadel, Ark etc) will help with the physical attacks. The Elemental Lord has a decent fire resistance so you might find
Adamantite and other anti-outsider weapons (Banishing as an attribute) do better than fire weapons. That depends a lot on your build, your traits and your weapons.
Supplies: Heal, buff and summon supplies will serve you well. You and your companion should drink potions like a pirate on frigul rum. That includes ice resistance potions, armour potions, attack potions, and your best summons. End-game healing potions are very useful here. Note: if you're taking the beating, a fire summons might do more damage, but if you need your summons to take the beating, earth, ice or iron summons will help. (There's an unique single-use item in the game that lets anyone summon an iron golem. Scrolls of Circle of Restoration are nifty, too.
Try to make sure you fight him alone. That means taking care of nearby enemies first and trying to keep from getting overwhelmed.
He's tough, but very beatable. And he's easy in comparison to the lazy boss hiding nearby.....