This is the way i did it mate.mmediately use the syringe on yourself. Now see the two dolls in her hand, you have to shoot the blue doll in her possession and this will release her grip on you permanently(they are like voodoo dolls), then pick it up. Then go for the red doll, doing the same and when she drops it pick it up. Once you have the red doll, equip it and aim it at her then shake the sixaxis like hell to fire at her, do this a few time and hit her and she will die. While doing this Meryl will keep coming at you so just knock her out by either using V-Ring shotgun rounds(the air ones) or grab her using a CQC attack then use the syringe on her, this will knock her out but only temporarily. If you get the blue doll(i think you can do the level without getting it) you can use it to control dead bodies, never tried it myself though i might later on. :Clap:
Your not shit at the game this is probably the most annoying boss lol