Just to sum up from another thread:
Elminster CANNOT (to my knowledge) be killed in BG1 as he does not recieve any damage. You may hit him alright and you may even poison him for all it matters. He just doesn't get a scratch.
http://www.dsr.kvl.dk/~maddog/Baldr014.jpg
I have tried this before and now once again just to show that it cannot be done. BGT is an entirely different game where you have access to spells and features unavaible in "true" BG1.
And what is the talk about killing the Demonknight on the ground floor? I have only met him in the mirror hall. Is there a procedure to get him to appear?