Attack her only from inside the purple circle at her feet. Unless it has changed colour with 5.1, but it's at her feet. If you are outside the circle, she reflects all incoming damage, so you end up hurting yourself.

Imprison Lord Dramath on Nathema. He comes out of the holocron and acts as a companion, and he will heal you.

Failing that, the green circles that wander around will heal you and hurt her. It is possible to kill her using only damage inflicted by these circles.

If you are determined to do it by beating her up, stay inside the circle unless she curls into a ball and puts out a red AoE zone. That skill takes about ten seconds to cast, at which point you will be knocked way far back, and probably off the edge of the allod(1), where you will be forcibly reminded of the correctness of HK-55's declaration that meatbags can't fly. Alternatively: 1. run outside the red circle. She cannot follow you while she is casting the skill. 2. position yourself so you are knocked back into the stairs. 3. Position yourself with a wall (the stairs?) between you and her, even if you are inside the red circle, since the skill seems to be unable to pass through the wall.

(1) "allod" = "island in the sky", and all outdoor parts of "Allods Online" take place on some sort of allod. Very few parts of SWTOR take place on allods, but that chapter is one, and the events on Makeb are another.