Okay, I'm in the ruins with the three portals and I got the song and dance quest on floor 13. 
But I simply cannot find the 13th floor!! 
 
I go through the portals and I find the 10th, the 12th and the 14th floor, but no 13. 
 
Please help?

You get to 13 by going through 12

There has to be something you're missing. Check the map thoroughly and be sure to explore every crack and crevice.

Each portal has two stages to it iirc - like Big says, run through 12 and take the teleporter to the Glyph Room or whatever it's called.

Thanks guys, I finally found him. He had spawned on the other side of the cracked floor in the glyph room. Took a good while to gun him down with a random grey pistol I had laying around my inventory =P