A long time ago I tried the baseball bat method and the only thing that happened was the tire absorbed the bat. It did nothing to the fender lip. Here's a method I have used with success, but beware, your mileage may vary: Using a decent size hammer (ball-peen, 24 oz) cover the hammer face with at least three layers of masking tape. Then remove the wheel and tire and starting at the front, hit the fender lip (inside, obviously) at a 45 degree angle. Hit it once, then move about a hammer's length rearward (3" or so) until you get to the rear of the wheelwell. Do this several times, using medium blows and keep checking your masking tape, re-taping if needed. I've done it without marking the paint, hopefully you can, too. Just don't go too far and be very careful to be accurate with your hammer, one missed hit and you may hit above the lip, in the inner wheelhouse which can easily show through on the outside.
