The wall pipe under the kitchen sink has a hole. I already disconnected the end that attaches to the trap, but the side that connects to pipe in the wall is badly corroded and the nut won't come loose. I already tried banging the pliers with a hammer and WD40. I can't get it to budge at all. Any suggestions would be greatly appreciated. I am worried about using a hack saw, because if I go to far I could damage the pipe in the wall...

What you need to do is hammer on the nut itself. Work you way around it hitting it fairly firmly with a hammer. Hit both sideways and length ways. Keep it soaked with the WD-40 the whole time. Heating with a torch and quenching it also works well. Once you have the nut loose, you may need to collapse the tubing and drive a chisel between it and the pipe going into the wall to remove it. Again, a torch might save some work. It also might set something on fire. Be careful about what else is near.