Lord_Casque_Noir: Zhar is not bugged. As StormyFacade said, he is only a bag full of PV which is hard to hit with weapons. But he is easier to kill with spells as he don't try to teleport as you are not near him.
Also Bow attacks work well on him. Counselors and those with the same AI, like Zhar, only "teleport" away when others enter melee range with them, or nearly melee range.
When he vanishes, you can still see the lit up tile he's on. When he teleports, he more or less just becomes invisible, gets like 3x faster than you until he stops, and also gains more armor, when in this mode, the tile he's on is still lit up, and you can still hit him.
The key is to back him into a corner, which can also be the tile the bookshelf itself is on. Ranged enemies and their AI break when in a corner, since their AI tells them to never move anywhere other than away from a melee attacked, and if they can't move according to that AI command, to then attack the enemy. Some enemies, such as Counselors, only ever attack you when not at low health, otherwise their AI tells them to simply run away when in melee range and nothing else.
I haven't seen the game's AI scripting, but it seems to be that way. My recommendation is to just take plenty of potions, get him in a corner, and whack at the "empty" air he lives in. High damage weapons help, also spell scrolls too like Inferno especially, firebolt's not that great, same with lightning, if you have the magic to cast them that is.
I mainly say only Inferno works because it does the highest DPS, since it's a damage over time spell like Lightning, but it moves slower, damages just as often when someone hits it and is perfect for those like a warrior, who are always wanting to be in melee range anyways, since Inferno only travels like 3-4 tiles from the caster.