Ah, well...
...now, what would make her stronger? Pretty much everything. Nature is bad, moveset is atrocious and would get rendered utterly useless if something with Flash Fire came along as your opponent - ironic since Rapidash gets Flash Fire too. A lot needs to be done here before this Rapidash can be at all effective.
Enough about that though, here's what you really need:
Rapidash @ Life Orb
200 Attack/52 Sp.Attack/252 Speed
Flash Fire
Hasty
- Flare Blitz
- Megahorn
- Return/Double-Edge/Hypnosis
- Overheat
Rapidash suffers badly from very shallow movepool, but it can make something of the tools it has. STAB Flare Blitz kicks ass and so does STAB Overheat to strike down physical walls, Megahorn is unfortunately the best coverage move it has though - hits hard though with 120 Power so it's still a good move, and the third slot can go to either another hard-hitting attack, or Hypnosis to put any Water-types trying to take you out to sleep.
Hypnosis, Double-Edge and the better Nature requires re-breeding unfortunately - if you don't wish to re-breed and start from scratch, either Return or Iron Tail is best in Slot 3 of your moveset.
I'd take a read through this topic here too:
http://www.pokecharms.com/forums/index.php?topic=3017.0
It'll tell you all the basics you need to know for making movesets and EV training. Definitely worth a look.