Princess Eilonwy... no one knows who she is so she'll be essentially ignored and can wait for the others to knock each other out before coming in at the end.
And then she can come in for the win once the others are weak and tired.
Thank you Wikipedia:
"Although having been trained as a sorceress and not a warrior, she is quite capable of using various kinds of weapons, particularly the sword, bow and spear. Even when unarmed, she will fight like a wildcat (i.e. scratching, biting, and kicking) until incapacitated."