Just go to the bomber and ask him, why he is there ?
Or even worse : Ask him a question/riddle/philosophical something that is so difficult that he will vanish like the two skeletons from early within the first Divinity 1 dungeon ?
Or something like ... making him believe that he doesn't exist at all (sililar to in PS:T making people believe the Nameless One exist - or like in the Discworld where things come into being by people simply believing in them ) ? - Would requite a persuasion skill check, I believe.
