I have found a forum that said that you could combine a poison paint with
a weapon and kill the head guard in one hit, but I cannot get it to combine with
ANY weapon. I am talking about the paint from the thief in the Huline village.

IIRC, you have to select the poison paint then click it on the weapon icon by your portrait. In addition, think it only works with bladed weapons.

Thank you, I will try that.