How do you defeat praying mantis in mgs4

The boss to which you are referring is Screaming Mantis. This boss battle has a number of stages. When gameplay starts after the cutscene, the first thing you need to do is equip the Syringe and inject yourself with it, otherwise you will be unable to use your weapons. Next, you'll see two dolls hanging from Screaming Mantis' arms, the one on her right being the Psycho Mantis doll, which allows her to control the living, and the one on her left being the Sorrow doll, which allows her to control the dead. She will use these dolls to control Meryl and the Haven Troopers that you either tranquilized or killed in the previous stage of gameplay. Use a sniper rifle (I recommend the M82A2, DSR-1, or M14EBR) to shoot at the Mantis doll. Each time you shoot it, it will stop the onslaught of Meryl and the Haven Troopers for a bit. As you shoot the doll more often, Screaming Mantis will progress to using Meryl to shoot you, and later to shoot herself. You can choose to tranquilize Meryl to stop this from happening. But if you continue to shoot the Mantis doll, this shouldn't be of any concern to you. Eventually, the Mantis doll will fall to ground. Run to pick it up before Screaming Mantis can pick it up. The next part may be optional depending on whether or not you killed any Haven Troopers in the part before the Screaming Mantis battle. If there are any dead Haven Troopers, Screaming Mantis can still control them with her Sorrow doll. Shoot this too until it falls to the ground and pick it up. If you did not have the Mantis doll in your inventory before, equip it (it will be found under Weapons). Then aim it at Screaming Mantis and press R1. You will then be given an onscreen prompt to shake your controller. Do this until Screaming Mantis' Life and Psyche gauge are empty (there is no way to non-lethally defeat her, so this won't count as a kill). After this, complete the Screaming Beauty portion and you are done. (if you are trying to collect the BB Corps dolls, Screaming Mantis' will be at the end of the hall where you first entered)