if i playing with my friend and he has archaon hiding in his knights and my stegedon comes charging hu do i hit archaon or the knights or do i divide equally
They will be assessed against the regular rank & file models. If there is a character (as in your case) you would randomize the hits. Pages 63 & 74 in the rulebook should clarify.
In combat you can choose who to direct attacks against of the opponents in base to base contact... In the case of impact hits, they are distributed as shooting I believe and generally just hit the bulk of the unit not any characters or unit champions that are there. This will be different if there are only a couple of knights though.