Using fsc from IDEA is supposed to be trivial task. IDEA provides dedicated fsc "Run Configuration" out-of-the-box. Unfortunately in reality things are getting more complicated. Scala build in IDEA is hanging more or less often making it not usable.
It took me several hours to figure out what is going on there. I was trying with dozen of switches without success. Finally I've found this simple answer and here it is.
When compiler server is starting it opens random port to listen for commands from compiler clients. Obviously the clients must know that port number. To communicate that server is creating file named as the port number. The file is created under JVM temporary directory ${java.io.tmpdir}/scala-devel/ ... (/tmp/scala-devel/{username}/scalac-compile-server-port/ at my linux box). Problems begin if there are some old unused files left in that directory. So the solution is easy - delete ${java.io.tmpdir}/scala-devel/ before starting compiler server.
I've created simple bash script to do that. I simply use it instead of starting fsc from IDEA.
