IMO Malgus is my favorite FP boss. I killed him like 10 times and never had a problem with being knocked off the ledge. You just have to be more careful when bringing him down for final knock back - move to the side of the stairs before moving down.
Near the ledge there are a few additional steps down. Don't go there, grenades and push backs should be enough to push him off the edge, instead position yourself, so his knockback doesn't throw you down.
Edit: For easy pull @15% I save both my taunts (slacking party members on stairs can be a pain), First I use AOE taunt, run down, then use my second taunt when I'm down the stairs.