If the top cap has any lip to it on the side, you can get a pair of vice grips on it. Then just turn the whole cap to the left, it should turn the bolt with it. As soon as you get it just a tad bit loose you should be able to turn the bolt with your fingers.
However, a top cap bolt should never strip. You shouldn't tighten it down with stripping force as this will damage the headset bearings. It should be tight enough to elimiate headset play but loose enough to have no resistance in steering.