How do you beat act 5 in metal gear solid 4?

Question

the part where your on the ship and you have to fight this lady that floats around with the hanging dolls

Answer 1

The guy above me got it totally right, I just wanted to add one thing.

Both dolls are based on characters from other Metal Gear games.

The orange doll (called the Psycho Mantis Doll) controls people that are alive. The Blue Doll (called the Sorrow doll) controls the dead. If you want to get the Sorrow doll too, just shoot at it after you get the Mantis doll. That way, you can control living and dead people next time you play through the game (since you keep all your weapons when you play through the game next time).

Answer 2

Its quite simple really.  She has the two dolls floating next to her.  One is orange and one is blue.  Just shoot the orange doll with you M4.  Each hit will break a string holding the doll.  Do this a few times and after two cutscenes Meryl will start shooting herself.  You have to stun Meryl before you can shoot the last string on the orange doll.  When the doll falls to the ground run over and grab it.  Then you must equip it and fire it at Mantis.  When you hit her hold L1 and tilt the six-axis towards you hard about 5 times.  

Thats it. Good luck.