I had to do this once to free my bike after losing the key to the padlock. Small pair of bolt cutters does most of the work, followed by a small pair of either diagonal cutters or linesman pliers to finish off. The bolt cutters cut through a majority of the steel fibers, but won't cut all the way. The wire cutters get the last remaining fibers. An easier way is a cordless dremel with a cut-off wheel, if you have one.