There was actually a bug in the DOS version that made it so that the difficulty level would always flip to Beginner after the first mission. The final patch (1.4) fixed it.
The PSX version lets you open doors without going through them. Take advantage of that. Also note that kneeling really does make a difference, both to your odds of getting hit, and to your odds of hitting your targets.
This post has been edited by Bomb Bloke: May 3 2009, 02:28 AM