I'd use either a air body saw or a 4.5" angle grinder with a cut-off wheel. With the saw, throw some masking tape over the whole lower portion of fender, then with a pencil, draw where you want to cut. The air saw is very controlled and the masking tape will keep from marring the paint. With the angle grinder (what I used when I still had sheet metal) the diameter of the wheel helps to make nice clean straight lines but it can wrinkle the paint from heat if you are not careful.
