As post from someone that can finish the Impossible, Powerful Magic, 4 opponents mode (so toughest).
Yeah, there is a
modification (actually
two) out there that allows multiplayer. I have a modification (at least not the second, maybe the first, I may have yet another...) and I have included a link where you can download it. It was not originally in the game.
What is the advantage of Archers over Mages and visa versa?
Archer can hit units with magic immunity - Sky Drake and Sorcery spells can give this.
Mages have no trouble with protections from arrows again. - like skeletons are immune, and some magic items give.
This site has loads of information to get started, and some insanely scores which you can read if you let the last wizard live as being quick does not yield as much points for your score as your inhabitants and stuff.