Sorry dude, but what youre asking is pretty crazy. Muscle weighs more then fat. So youd have to gain a little. Also, fat slows you down!
It sounds like your looking for something to help your endurance, Id say ummm cardio work? Cut back on the weights a bit and try some running. Theres no miracle supplement to help that. Supplements only help, if you take creatine you better train hard cause if not youll just waste it.
Also, boxers, train on heavy bags a lot. That builds power. They run a lot and weight train. My best advice is to get a book on boxing and boxing fitness.
http://www.bodybuilding.com/fun/bbinfo.php?page=Boxing
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