Cheats
How Psycho Mantis Works in the MGS Master Collection
Just like in the original PSOne classic, Psycho Mantis is a boss that uses mind tricks to read Snake's thoughts and predict his actions. But things have changed since the PSOne days, and the clever ways in which he did those tricks and the ways in which you would defend against them are no longer possible. Thankfully the Collection accounts for that.
To have Psycho Mantis read your save data, in the main modern menu select "Edit Saved Data." There you can create a false PSOne memory card with saves from all of the games that Psycho Mantis would have unique dialogue for.
More important to note is how to defend against his reading of your movements. In the original version, you'd do this by switching from the player one controller port to the player two one, but since controller ports no longer exist, you'll have to use some extra trickery.
Press the home button on whatever console you're playing on, and then press it again to force the game into a stance menu. From there, you're able to go to controller settings, switch the controller connection option from P1 to P2, and you'll be good to go