How do you deter an Unwanted Cat, to my own Cats garden?

My peace loving cat rules the roost (my large walled back garden), but has recently been bullied/stalked by a beast-like panther cat, who stalks her every few nights/weeks. How would I best deter this intruder so to aide my cats often stressful expereince overseeing the garden/birds & bees? My mum suggested (quite cunningly) to take a leak in the garden, to which my own cat will be used too, yet the other one won't. whats your view on this? No nasty messages either....! I am big cat lover, oh yeah, and iI'm a guy, so no squatting needed in the garden!