I use a lot of mulch and compost so I provide an excellent hiding place for slugs and snails, but I have had pretty good luck using diluted ammonia with a pump sprayer. It does have to be repeated, but how often depends on the severity of the infestation and the weather. I usually gauge how often I spray by the number of slugs or snails I see, or by the damage to the plants I find. I don't think the ratio is really very specific, but I think some where around 10 to 15 parts water to one part ammonia, you might even dilute it more just trial and error mostly. I have sprayed it all over the plants and have seen no damage to them, I tell myself I am adding a little nitrogen to their diet as I kill the slugs and snails. I use the spray on the hostas and the daylilies.
Maybe start out with a very diluted mix, spray some on a visible slug or snail and see how it reacts. I like to spray the entire plant and the soil beneath it to try and kill the eggs also.
