I once had to cut a similar piece for an oak mantle I was building - laid out the arc with a flexible batten, then cut it with a jig sa (and a new blade). Then, cleaned it up using an orbital sander unti it looked right.
I'm getting ready to build another similar mantle (this time for my ow home) and will probably use the same technique. If this is somethin that you will be reproducing in the future, you may want to conside making a template from 1/4 inch hardboard. Just make sure that yo pre-cut the arc to within 1/8" to 1/16" of the line with your jig saw then finish it off with a pattern cutting bit (bearing on top) in you router.
Regardless of how you finish your cut, make sure that you are using good blade in your jigsaw
-- makesawdust