as a tank that's ran this probably 20 times, without being knocked off once I can tell you without a doubt that OP is doing something wrong.
keep your back to the wall at the side of the stairs until 15 percent or so, then move him towards the ledge with your back towards the long hallway, if he knocks you back you wont fall off. when he starts casting lighting force push him off the edge. its INCREDIBLY easy, and there is no excuse for being knocked off.
seriously, if I ran as dps with a tank that got knocked off I'd kick him from group and find another.
only thing wrong with this fight is it's too easy. he should hit harder, and cast unlimited power more often.