I'm not sure that there is much you can do to help your cat fight back. As others have said, spraying it with water will dissuade it to an extent, but ultimately it needs your cat to assert herself a bit more, and at least make the other cats life difficult enough that it stays out of the garden when she's around.
Talisker isn't terribly territorial, or inclined to fight other cats, but since he's been here, he has been guarding the back gate, which I think a number of the local cats had been using as a convenient shortcut from the front of the houses to the back gardens. He's obviously had at least one aggressive fight, since he lost a bit of his ear, but generally these sort of fights don't result in much injury.
If you have any issues with the other cat coming into your house, you probably do want to try and stop that, since that could excessively stress your cat. Either a conventional cat flap with a magnetic lock, or better an RF ID flap, such as the Pet Porte will stop that.