The water-pistol method: I have a water pistol (a spray-cleaning bottle) which I have filled with salty water. Every morning and evening, when doing my stroll around the garden, I spray any of the creatures I see. I also mist plants that are being attacked, even though there are no slugs visible. Brassicas are well able to tolerate some salt - I think they are descended from a coastal plant - and it has never done any harm to any other varieties that I can see.
The good news is that after a couple of seasons of this treatment, I have no trouble at all with slugs and snails.
In the long-term I may have to deal with a salt build-up but the slug-free years will have been worth it.