I found this trick some time ago and never took a photo of it. I'm posting it now since I got to use it again this weekend. I've seen various tools mentioned (chains, too). I found this to be easier for me, and doesn't require any extra tools or chains - just a single 13mm wrench. Basically, put the box end of a combination wrench on a WP bolt across the underside of the pulley and allow it to be wedged between the fan nut and the crank pulley. Turn the fan nut until the wrench rests against the crank pulley. You should now be able to loosen the fan nut without any slippage as the wrench holds the pump pulley very securely.