So very easy mate. Start with 4 spheres and a raytrace material with 100% raytrace & 10 recursions.
Place 3 spheres in a triangle, all on the same Z level. Then place another sphere on top of these, so you have a pyramid.
Then move the camera to view the middle of all four spheres. (In fact what is actually the space between all 4.)
Add a plane just below the 3 base spheres, make this an emitter ( after a few tests I reduced this to 0.4) add an image to the world with AngMap and another to the emmiter plane. (This gives the fire fx in the middle.)
Then add more spheres and other objects. If everything is aligned correctly to the camera (X direction) then you’ll get symetry. I mis-aligned a couple of things fractionally, to give a hint of variation.
No lamps or AO, just the emitter plane and Radiosity turned on in the render buttons. 30min @ 1600x1600 8 samples or 42mins @ 16 samples.
Hope this helps,