Now you're in the Underground Base. Crawl through the vent. There are two sentries on the wall and one sentry patrolling. This is another area easily bypassed with a chaff grenade. If you shoot a robot - even as a caution mode - a whole bunch of scarabs will come up from those holes in the floor and swarm the area. Move quickly and shadow the patroller. Watch for him turning around and scanning behind himself suddenly. Cross the bridge - avoiding the lasers - and move to the right. Go up the stairs and collect the ammo and supplies. Wait for the robot guard to head south toward where you came in. Move toward the door (watch out for the sentry right above it) and head in.

This is the underground supply tunnel. There's two fights here. First, Vamp. He'll jump around, slash at you with knives, and throw homing knives at you. Hit him with a rapid-fire weapon, like a machine gun or the six-barrel grenade launcher you got from Raging Raven. Once he's down (through lethal or nonlethal means) get behind him and use CQC on him. Equip the syringe item and hit triangle. That'll take him down. Now you have to deal with a bunch of Gekkos. You need to occupy them for as long as it takes Raiden to fight Vamp. The right half of the screen is the knife fight, so your visibility is limited. Back up and use the railgun to pick at their heads, then use machine guns to keep them down. The ones with flashing lights are your main priorities. Don't bother trying to charge up; two or three low-charged rounds through the head will take each Gekko out. Be aware that they can and will jump at you and crush you, so stay mobile. Basically, when you're not shooting, you should be charging the railgun for the next shot. While you're charging, you can take the time to watch the fight on the other side of the screen. Stay low and behind cover until a Suicide Gekko shows up, or the other Gekkos will hit you with their machine guns. As the fight goes on, the Gekkos will start having shorter and shorter detonation sequences. When the fighting dies down, collect the supplies in the area (ammo and a ration).

Now you're controlling Metal Gear Rex (minus its nuclear railgun). Rex has three weapons: gatling guns, missiles, and a laser.. Gatling guns should be your primary weapon for this part. Missiles and lasers take a long time to charge and the guns do as much damage as they do, so just stick with it. Sweep across the waves of Gekkos. If they crowd you, hit R1 (without L1) to sweep with your feet. Hit triangle and Raiden will help you out. Stand back when the Gekkos explode or they'll damage Rex. Bust through to the port area and you'll have to fight Metal Gear Ray. The best strategy here is to hang back and charge your laser while dodging his attacks. If he gets close, or launches missiles, rake him/them with the Gatlings. If he stands still right in front of you, launch all the missiles you can at him. If he charges at you, press X plus a direction to sidestep and then counterattack with triangle. If you can get close to it after knocking it down with a missile barrage or laser hit, hit R1 (without L1) to kick it around, then when it's down use triangle to deliver a major attack. Stay in open areas so you have room to maneuever. Dodge with X to avoid both Ray's charges and its water-cutter laser attack. Keep at it and Ray will go down.

Metal Gear Solid 4 Walkthrough