How to Beat Screaming Mantis in Metal Gear Solid 4

Prepare. Make sure you have the M4 equipped with plenty of ammo. Even if this is a no kills run do this, you won't harm your efforts. The firing rate should be set to automatic. Buy a scope for your M4 if you haven't already, again vital, it should cost you 17500DP. If you can't afford this use the M14 if available and set to semi-auto. Buy a grip if you can, this will greatly benefit you. Now equip the Mosin Nagant, if you don't have it equip the M870 Custom with ring shots, or stun grenades. Failing all those just use the syringe, it is the most effective means (will be explained) but the most dangerous.

Immediately syringe yourself (L2 to syringe and X) and then switch to rations if this isn't a BBE or other elite emblem run. If it is, then switch to the Solid Eye for the time being. The Syringe is acceptable to use in an emblem run.

Run straight forwards and up the stairs to the right ignoring all the Haven troopers. If this is your first time and its not an emblem run by all means take down any that get in your way from this point, however, if it's an emblem run remember they all count as kills even the dead ones (ones that fall and die when you tranquilize). Once your on the top viewing area, check all the passages for rations, there's a total of 3, one of which is next to a column to the right. If you don't need rations continue.

If it's your first run, use the M4 to attack both dolls, either is good.

If it's a return attempt; use your M4 on the Mantis doll (red doll) only.

Scope. The scope will make everything easier, also going to options and making the movement speed of everything to 10 is a great help. Keep firing at your doll, and when she screams "Snake" drop the scope view and run. She will then fly at you, and miss, use this opportunity to fire at the dolls unprotected. She may do 2 attacks in sequence, be ready for that. She will also fire blades, there's not a lot you can do but try to avoid them. They do major damage in Extreme mode.

Eventually she will make Meryl attack you via the medium of cut scene. This is phase 2. Take Meryl down with the M870, or if you can get to her, grab her round the neck and jam the syringe into her. Then take this opportunity to attack Mantis' dolls further. Keep running from Meryl, keep firing and preventing Mantis from attacking. During this phase she will frequently use Meryl and Haven soldiers to defend the dolls. Shooting them in the foot once or twice shouldn't kill them, if you don't mind killing Mantis soldiers, kill them and keep firing through you'll likely hit the doll. The tranquilize weapons might be a bit slow but if your aims good by all means use that to move the bodies.

A second cut scene should eventually begin where Meryl is forced to attempt to take her own life. Fire at Meryl with the Mosin Nagant. If you don't have the Mosin Nagant, take her down with the M870 or the syringe (grab and stab). Make sure her stamina bar (psyche is you will) in at 0 before you resume your attack of the dolls. She will use bodies to defend the dolls less now.

Eventually she will drop a doll, immediately drop your scope and run for the doll. Once you have the doll; assuming its your first time, you should return to the top viewing area and resume attacking the second and final doll until she drops it. In extreme mode she can re-spawn dolls. If you found taking the doll down easy, this is good way to get DP. However if this is your first run you won't be able to do this. If it's a second or further run, you should have just taken down the Mantis doll. If you wish you can take down the second doll for DP. Otherwise attack Mantis with her doll, hold L1 and shake the controller while she jerks around until her life bar hits 0.

If it was your first run, proceed to take down the second doll, and then use the Mantis doll on Mantis, as previously explained (Hold L1, and shake). There won't be any further cut scenes, until shes dead.