She didn't know what she could have done without Dan, probably would've lost her brother in that fight.
Truthfully, she didn't go to fight the mandurugo unprepared, she had a secret upper hand, fat thanks to Dan.
The Mandurugo feeds on human blood but for unknown reasons was hooked on her's, but she's clearly not human as said by Dan.
She wouldn't boost of having a high IQ but she was smart and understood at once what he was implying.
So she pulled a weak front, appearing scared and submissive tricking the mandurugo into thinking she was desperate to save the life of her brother, so therefore would do anything.
And yes, she was desperate to save her brother but the real reason was for the creature to let down its guard. So when it finally did, boom! she striked and defeated it or so she thought.
