The general idea is first to use the syringe item, which will break Mantis' psychic control. Next, you want to avoid her attacks, while shooting the dolls floating near her. If she starts to control Meryl or anyone else, tranquilize them to break the control. If you need more detail, calling you contacts can give additional tips, or here's a guide that explains it step-for-step.

http://guides.ign.com/guides/714044/page_81.html