[quote]A target with [b]total cover[/b] can't be targeted directly by an attack or a spell, although some spells can reach suc a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.[/quote]
- PHB p.196

Thus, to hit an arcing shot they would have to determine targeting "randomly" by choosing a five foot square behind hte wall to aim at, which would be an attack against the space, not the person. If the person happened to be in said space, sure the attack would hit (and crit with that 20). In this case, making a direct attack like that is not a viable, nor possible course of action.