put a adjustable wrench on the brass nut just below the spindle .
put another pipe wrench on the body of the cartridge. I would wrap something like lots of duct tape so not to screw up the threads on the cartridge. then turn ever so slowly.
Thanks, Bill. Turns out I need a bench vise to get the job done. Are Kohler parts like in the pic interchangeable? I was on the Kohler site and found what looks to be a similar looking part and it was the only one they offered.