Starbeam is right. Generally a lot of guitar players have two guitars on hand already tuned. I know one who works this way. He has... Actually he uses a total of four guitars. Two are tuned to A. The other two to E. Nobody wants to tune and re-tune a la Bob Dylan circa '66.

   I love the idea you would want to use a guitar in your story, Umpty. If I could give a little suggestion, of course this being your story. I personally wouldn't make the whole guitar indestructible but give it some special powers that even the owner wouldn't know about. A spell in which cast, but the main thing would be to figure out what exactly is so magical about the thing. It could be a string, a fret, or something as little as the knob. Then get yourself a guide about guitars so maybe the hero can take it apart and wonder which part of it is magical. An electric guitar would be best because it's solid body if you really want to smash somebody or something with it.

   The two wrestlers who would use a guitar to smash over their opponents heads would be The Honky Tonk Man and later on Jeff Jarrett.

    delphinerose