Multiple saves and shadowfields

If you roll multiple saves on a shadow field (which stops working when you fail a save), do you roll one save at a time or roll them all at once, and if you roll them all at once do all the wounds go through if you fail one or does the shadow field save all but the failed ones and then is not available when wounds are taken from then on? For example my archon takes four wounds and fails one save, does he only take one wound or does he take all four?