Yes but she can be in the middle behind 2 better defenders in advanced defense formation (Double), then you will have only middle line of tiles to hit her. Stacking works with much lower success in this case.
Especially when one of the front heroes is lets say RED (Mirjana) the other lets say BLUE (Richard).
Brynhild + Season one only heroes. Add costumes and / or emblems and it becomes pretty annoying.
Sniper Brynhild Sniper
Marijana (Gormek) Richard
Not saying is impossible to beat there is no such defense but there is a reason some heroes affect three. The advanced defense formation make these heroes quite more powerful.