That would depend on how clean of a cut you need. A plasma torch would be about the best way I can think of. I don't personally own one, but have used one a bunch of times and as Wayne Cambell (Wayne's World) said, "She will be mine....oh yes....she WILL be mine"
Otherwise I think you're pretty much stuck with a Sawzall (buy the expensive blades....they make a difference) or a grinder (thin cutting wheels are MUCH faster).
Why exactly are you hacking it up anyway?