Lazar creates a mirror image of himself up constantly, and it can be difficult to tell the difference unless you're dealing significant damage. The way to tell is his mirror image will take double damage. Keep attacking whichever image is closest to you if you can't tell.
Lazar sometimes teleports onto the waterfall on the west side of the room, and will not get out. He can still be killed by ranged attacks. Use the Hunting skill Reveal to see him.