This is about the series - Game Of Thrones.
They show that the Red Woman can use magical powers to infiltrate into Renly Baratheon's quarters and kill him with quite the ease. She doesn't fight the guards, she bypasses them in her "shadow" form and heads directly for Renly.
If that is the case, why does Stannis Baratheon not use her to sneak up and take out all the key players of King's Landing and claim his throne?