Impervious Toughness seems to be a common case in M&Ms to represent characters with great resistance to damage. But how do you buy Impervious for Toughness? Impervious is an Extra with +1/rank cost, but you cannot buy ranks of Toughness, only increase it by buying ranks for your Stamina or the Protection power.
Where do I add that +1/rank cost? To my stamina cost?