Have a dying survivor on th ground tell you some survival tips and some hints about rocking the balance of certain enemies. Or do a sparring seqeuence in the beginning of the game where you have each character spar with someone, (outlander and his tirbe chief/d'arce and the captain/mercenary and his bandit captain trainer/enki and his cult)
Yeah it's definitely a mechanic that needs to be explained.... Although in the upcoming version the chances of hitting the head are closer to the chances you had in the previous versions, before the weakness mechanic that is. So rocking the balance is just a guaranteed way to get that headshot, not the only way.