Please see below for an explanation.
Main difficulty with the Keeper in Ancient Labyrinth DLC is his deadly AOE spell, which you can only survive behind a wall or behind a magic decoy (Prayer).
I found the method shown in this video is the easiest to handle.
Please see video at 2:00 for details, explanation is there.
There are 3 key points in this video, being at 2:00, 4:40 and 6:58.
Good luck!
