[Ruy Dias de Bivar]

Simple!
Shotgun + rock salt.
Shotgun + fine bird shot.

After these two fail to stop your attacker, Shotgun + buttstroke.
Next, your are too late to use the buckshot loads.

[fini]

I agree. Rock salt or #9 birdshot. Go for the legs, but don't worry about it too much. Rock salt is probably best, because it is light (so the recoil will be less). Usually just the noise from one shot blast will cause all but hardened men to flee.

When I was in my teens, we had a real problem with trespassers. I kept my double barrel shotgun with a shell with no shot (just noise) and the other with #8 shot. Since it had a single trigger, there was no problem making an accident. I had to shoot the "no shot" shell a couple of times, but never the #8 shot.

If they really want you, then they can probably figure out how to do it. But most will move to easier pickings.