If the wizard and fighter are separated by enough of a degree that the wizard can't be helped on his own, then something probably went wrong, but further than that, wizards are expressly designed to not be as martially capable as a fighter. The mere fact that you have classes should inform how much "roles" are considered an integral assumption of the game. And beyond that, expecting the casters to be able to overtake the martials in their given roles is an unhealthy approach to things, because it doesn't consider the impossible inverse. If wizard and clerics can just take over what a fighter or paladin is doing, then what the fuck is the point of the fighter or paladin? It's not like they can turn around and do the wizards job if they end up separated.