Basic rule of thumb is no one can hurt you if you don't want to take the hit, though it is generally frowned upon if you are in the middle of combat and taking absolutely no damage whatsoever. A grazing wound here and there never hurts, but yeah. Likewise, you cannot call a hit on anyone else. The only instance of this that should happen is if you message the opposing writer OOCly and establish the results of the fight between you and a character. There is no dice roll system or any hard set of rules as far as combat goes, just don't godmod/powergame and you will be fine.