Quote: "Doe's anybody knows how to make an terragen terrain to an FPS ready terrain."
Many people have tried (keep in mind, unless you own the commercial version of Terregen, you cannot use it in commercial ventures); but do the the high poly count of the mesh (and some of the export problems), it was deemed not possible. However, if you could import it into a poly reducer, it is plausible that it could work.
Quote: "second thing is how to make invisible walls that will not show bullet hole when they are shot at or blood stain when an enemy is killed near them"
Just make a segment with a completely black texture (0,0,0); then figure out how to remove the decals from the segment (my guess anyway).
-Keith