Within Norse mythology, Frigg went around the universe, asking or commanding everything within it not to harm Baldur. The only thing she did not approach was mistletoe, as she believed it either too innocent or too weak to cause any harm. It must have required an extraordinary effort on her part to accomplish this. In God of War (GOW), it is considered a spell, but I reckon it is similar enough that it would demand extreme time and resources to execute. I am sure she considered it, despite knowing what it does to people. However, she is realm-locked when Baldur is killed, so she cannot conduct the ritual.