How do you fight the Black Thing in the book A Wrinkle in Time?

You fight the Black Thing with love.

The Black Thing is a manifestation of evil.  It overshadows Earth and other planets in
the universe, and it currently has Megs father captive on the planet Camazotz in the form of
IT.  IT is a bodiless brain who controls the entire planet, where everyone is in lockstep and
lives in fear of not conforming.  When Mrs. Who, Mrs. Whatsit, and Mrs. Which take the children
there, Charles Wallace falls under ITs spell, and now Meg has a problem.  Her father and her
brother both have been taken captiveone literally, and one mentally.

Meg is
able to get her father free using a trick (Mrs. Whos glasses) and her knowledge of science. She
uses her unique way of looking at the world and her love for her father.  She compares it to a
rice curtain at a beach house.

At first Meg had flinched
each time she came up to the curtain; but gradually she got used to it and would go running
right through, leaving the long strands of rice swinging...