I second the other advice about getting a vet to look at this. Painkillers are the least of your worries; if there is dirt or debris stuck in the layers of this wound they will need to be cleaned out by a vet. There is no muscle on this part of a horse's leg and circulation is very poor compared to a cut on your own leg or arm. Also, compared to our own injuries, some things that seem to make sense to us, like forceful streams of water to flush out dirt, hydrogen peroxide and some medicines can cause more delay in healing and do more harm than good.
Take a look at this thread: Phoenix had an accident - (graphic contents)
It's long and has graphic pictures, but is very educational about what another HF poster went through with her mare and a similar injury.