Kalinda wasn't stupid. When she was in the shadows, she couldn't be seen, and she couldn't be grabbed. So she stayed in the shadows, using the shadows from the plants and from Flood and Nicholas, to get close to Nicholas. She was right near him, and he wouldn't know, the shadows effectively covering her. she was never sure how it worked, but the simplest explination was that she just became the shadows. And you couldn't touch shadows. 

She gathered all her power, and flung it at Nicholas in a telekinetic attack