Work out each combat one at a time - resolve the fighting for all of the troops involved in one combat before moving on to the next combat.
A combat is often a fight between a single unit of troops from each side, but it is possible that several units, monsters and Heroes may become involved. In the case of several combats being fought at once, the player whose turn it is determines the order combats are resolved.
So long as fighting units are interconnected, they are participating in the same combat. All the combat results for such a combat are worked out at the same time.