Any time you use EQ to try isolating things it'll affect the other instruments because there's always some overlap in overtones. That's why trying to use a noise reduction or vocal isolation plug-in makes everything else sound worse.

If you can get the full track and a track that is missing one part (like a track without vocals or without guitar, etc.), you can try using phase cancellation. Line the tracks up as close to perfectly as you can, invert the phase on one, and combine them. That should isolate whatever was missing from the second track (the vocals or guitar etc.).