First Merryl will be controlled,so try not to shoot her. The main goal is to shoot the the dolls.If you have a sniper rifle that will be helpful.Moments later, Merryl is about to kill herself(It is not true). When you saw a doll fall,it will worth 10,000 DP.(IT should be the mantis doll so you can attack her). And thats how you beat screaming mantis