Things You'll Need
Instructions
Inject the syringe into yourself when the cut scene ends and the fight starts. This will give you better control over your character and help you aim your gun properly when shooting at the dolls.
Dodge enemy attacks, and once you are far enough away from the enemies that they are not immediately attacking, aim at the two dolls on either side of Screaming Mantis. Shoot the dolls several times until you see Meryl start to move to attack you.
Hide behind the pillars at the top of the stairs. According the GameFAQs.com, this is the best position because Meryl will leap past you and you can grab her from behind as you would if you were interrogating a character. Do not kill her, but inject her with the syringe to knock her out, keeping her from attacking you and putting her out of harm's way.
Shoot the dolls, dodging any attacks Screaming Mantis throws at you, until Meryl revives. Grab her from behind like before and inject her again.
Continue shooting the dolls. When one falls to the ground, run and grab it before Screaming Mantis can pick it back up. Once you retrieve the glowing orange doll floating on her left side, equip it and shoot it at Screaming Mantis. The onscreen instructions will instruct you how to use the doll to kill Screaming Mantis. According to GameFAQ's, you will press "L1" after hitting her with the doll and then move the control to shake Screaming Mantis around and kill her.