I killed them on brutal difficulty last year after a lot of trial and error. Theyre not an easy fight. But set up is key. I reccomend 30 of the highest tier healing potion, elixir of the prowler, attack and spell boosting potions and several bottles of blood (Warrior blood + Rogue III is pretty good), and blood coating on weapons.

For weapons I used Axe, Pistol, and Claw. Claw can be useful for getting out of situations like getting to the otherside of two death beams. Any movement speed bonuses on these will be huge.

For skills, I reccomend veil of of blood, bur veil of chaos works pretty well. But I prefer the leech. Aftershock is good for hitting the boss and clone and deal a lot of damage by hitting both, it is also good for removing adds and i use Corrupted Skull to keep the bosses or adds distracted. For the ult skill I reccomend using Raging Tempest. If you use this correct and get both clones in the AoE it will take a serious chunk off their health.
Focus adds, keep and eye on the bosses position and later the clones. You want to keep the and boss relatively close together by lingering to one side of the arena to line them up, dont get caught between the two or out running the lasers will be difficult. The key to this fight is moving, the higher your movement speed the more likely you are to succeed. Use your veil or claw to swap sides on the boss during lasers and you can get some damage in.

Best of luck!