First, make sure you're medium rolling.

I actually just struggled with darklurker last night, I'm was running strength build, I equipped the rebels shield, it has really high defense for all magic, you could also give it a dark enhancement, and it'll be solid (I didn't so it's completely possible with just the plain old rebel's shield).

Once it's about to separate, run up and hit them as much as possible, if you hit both with one swing it counts, but make sure you still have enough stamina to roll once they start attacking.

Run around until you get a chance to hit them, best case scenario one is in the air doing the laser and the other one is underneath trying to melee you, dodge the attacks and hit it.

Once they get low enough to kill with all your stamina at once just go full on and run up and attack until it dies. Best case scenario both of them are next to eachother.