How to Remove a Stuck Kitchen Sink Strainer
I'm assuming you are referring to the large nut that compresses the drain basket to the sink. Grasp the tailpiece with some pump pliers and use a large wrench or second pair of pliers to turn the nut.  If all else fails, cut the nut off and replace the tail piece and nut assembly..  Be sure to use some plumbers putty around the basket flange before you reinstall it into the basin.  Tighten well so most of the putty squeezes out.