Do you need to make the ends straight or is it the top face thats not flush?
If you are making the ends straight and you have a circular saw, just run the saw along a guide rail or if not Festool, run it against a straight edge clamped to your workpiece.
If you are making the ends straight and can run your router on a guide rail, just use a straight bit and take several shallow passes. If you can take most of the proud part off, say down to 1/16th or so with a hand saw, then a straight bit with a bearing could work if you can attach a straight piece to your workpiece (nail, screw, doublestick tape...). In either case, its a good idea to attach a sacrificial piece at the end of the cut, so your tearout will be on the sacrificial piece and not your workpiece.
If you are trying to make the top flush, a sander is the best choice.
Hope this helps.