[Lord Brass]

The Doomsayer is only interested in the idol, not you - or anyone else for that matter. If you don't have it then it'll go into the tomb and find it. If you want to engage the Doomsayer for the XP (and nothing else I'm afraid) then you must carry the idol somewhere about your person. The ghost will not target that character to the exclusion of all others, so you can "tag-team" it. I've killed the thing with only three characters, but it was tough. It was potions of speed, potions of healing and multiple magic missiles for me, but there are a variety of methods.

As for killing the archaeologist, it depends if you're a bad guy or not. If you accept the task from the guy outside, whose name escapes me, then the only additional person you'll have to kill is the archaeologist for which you'll lose 5 Rep. Points. Go outside and talk to the NPC again. As soon as he's finished talking, kill him. This is important for two reasons. Firstly, if you want the idol you'll have to take it off of him (you have to make a conscious decision to carry it). Secondly, if you kill him before talking, you won't get your 900 gold (which should be more than sufficient to replace the 5 Rep. you lose and still have money over); the money isn't on his corpse if you kill him - only something like 48gp or thereabouts.

You'll need the idol either way if you want to fight the Doomsayer, otherwise it'll disappear into the tomb and you won't be able to destroy it.