Developing Your Cardio

What's the best way to develop your cardio for a specific activity? The answer is doing that activity. I'm not say that running, biking, swimming, eliptical, etc. is not good. You have to do those things to supplement your primary activity. If a fighter is getting ready for a fight, the main form of training is sparring. Of course, he or she has to shadow box, hit the bag, pads, etc. But, the primary training method is specific to the actual activity.