So far (chap 6) the toughest by far was the Demi-Lich if you don't know ahead of time how to kill him. If you read the boards, it was easy, but NOBODY who has played this game w/o prior knowledge could have beaten him on the first try w/o losing chars to imprisonment. If you say you did, your nose is growing REALLY fast...

Arkanis Gath seems to be the toughest dude in the game.

He kills all of my characters with one hit each. I remember one of the shadow thieves saying that there are poisons that can kill you on contact, but this stuff is so powerful it puts out even my cavalier who is supposed to be immune to all poisons. Now what’s up with that?

This seems to be one of those cases where irritating the Dungeon Master results in a death dealing bolt of lightning from the sky for trying to ruin the plot, or is this guy really that tough?

When trouble starts in the shadow thieves’ guild, he suddenly appears and starts cutting a swath of instant death.

One other tough guy I found was the captain of the guard outside Nalia’s keep, after you finish liberating the stronghold. I pounded on that guy for like 15 minutes, but he never went down. He never killed any of my characters so he’s not as tough as Gath, but he didn’t eat dirt either so he’s tougher than any dragons I’ve met.

I haven't gotten to the demi-lich yet, but at least people seem to be able to kill him. I'd like to know if anyone has killed Arkanis Gath. He's in Chapter 3 in the shadow theives guild and I'd put him up against the demi-lich any day. If you can kill Arkanis Gath, I'll change my tune. Just go to the shadow theives guild and start killing people, then Arkanis will appear. Good luck and do a quick save first before you start causing trouble.

Arkanis Gath also pissed me off. I don't think you can kill him (see exception below).
He seems to do instant death. He never misses and each hit kills the character with no save possible. Once he kills the protagonist it is game over.

I spent about 1/2 hour trying various combinations and strategies to kill him. But he never got beyond 'Near Death'.
I think he's the method by which the game says 'sorry you just made a decision that will prohibit you from finishing the game'.

He appears if you piss of thieve's guild and if you're following the path of siding with the thieves guild (this includes the Maeron guild quest even if you haven't paid the 15000 gp fee yet).

I think this was a poor decision by the game designers. Especially in the Maeron guild quest. It should be possible to kill them all, find the key and then report back that you have evidence. But the game prohibits this by throwing in Arkanis Gath.
Lame.

Similarly, if you elect to side with the vampires and later attack Bohdi she kills the entire party with a single spell. This is a bit more definitive because your party doesn't even get a chance to attack back.

NOTE: if you side with Bodhi one of the first quests for her is to hijack a thieves guild shipment. In that encounter you get to fight Arkanis Gath and he is not Mr. Invincible anymore. So along that path you can actually kill him (but at the price of siding with the vampires)

those Pit Fiends come in very handy at times like that...it is fun to watch the enemy be torn apart like that, after spending too long on one guy!...