Several direct blows to the drain plug with a 2lb hammer will aide at breaking the static tension between the plug, the copper washer and the pan. Once you have introduced the hammer to this dreaded rounded off beast, the vise-grips will be the weapon of choice to finish loosening this monster! If you are unable to extract the monster than I believe fusing another bolt to it would be the final round before one would concede to extreme options.

For the future: Prior to placing a wrench on the drain plug try hitting it several times with a good size hammer (brass whenevr possible), it helps. It does not matter whether the drain plug is an internal (allen type) or an external (bolt head type) you will avoid overstraining yourself and then leading to an injury. Nothing better than flat backing on the drive-way, or in the garage, your hands and arms in tight quarters and then peeling off some skin from the knuckles because the wrench slipped! F#*@