Beating Psycho Mantis in "Metal Gear Solid" requires you to break the fourth wall and overcome the psychic's tricks by moving your controller to another port. With this solution in hand, the fight takes very little effort, but you should still stock up on a few basics before entering the battle: Take as much SOCOM or FAMAS ammo as you can find, a few stun grenades and a couple of rations for when you get hit.
Dealing With Meryl
Before the fight with Mantis even begins, you need to stop Meryl's attack. Take the Colonel's suggestion and knock her out: Run around her to dodge her shots, then punch her or roll through her repeatedly until she falls down. If you have multiple stun grenades, throw one to take her down instantly. Meryl might recover during the boss battle and attack a second time. If this happens, just stun her again, making sure to save at least one stun grenade for later in the fight.
Shielding Your Mind
Mantis' psychic powers allow him to dodge every shot you fire. The trick to hitting him lies outside the game: You need to change your controller port. If you're playing on the PlayStation 1 or playing "The Twin Snakes" on GameCube, pause the game, unplug your controller and move it to a different port. On the Vita, PSP or PS3, you have to change the controller port through the settings menu. For Vita, tap and hold the center of the screen, then press "Controller Settings" and "Assign Controllers." To reach the menu on the PSP or PS3, press the "Home" or "PS" button, respectively. The PC version of the game completely bypasses this gimmick if you play using the keyboard. If you have a controller plugged in, however, you need to take it out and fight Psycho Mantis with the keyboard alone. After you've made the change, Mantis won't dodge your bullets.
Fighting Psycho Mantis
Once you've dealt with Mantis' tricks, the actual fight is straightforward. Throughout the battle, Mantis pretends to turn off your video screen, but the image reappears after a few seconds. To attack Mantis, use the SOCOM or FAMAS -- explosive weapons risk hitting Meryl, and Mantis doesn't have much health anyway. The boss room also contains some extra SOCOM ammo. Mantis has two moves to watch out for. First, when Mantis turns invisible, equip the thermal goggles to find him. Mantis' primary attack uses furniture as missiles. Run from side to side to dodge it, and then resume firing when he stops his assault. Some of the patterns are hard to dodge, so heal up with rations when your health gets low.
Ending the Fight
After you take down most of Psycho Mantis' health, he revives Meryl and tries to make her shoot herself. This is the moment you saved the stun grenade for: throw it, knock her out and resume shooting Mantis. If you don't have any stun grenades, you have to punch her out as quickly as possible. Either way, Mantis falls after a few more shots, and you can move on to the cave.