I'm not sure how alkaline a soil hostas can stand, but if I was at my wit's-end with slugs I'd try dressing the stem areas with paver base. It dries quickly and its little particles have sharp edges, but it doesn't disappear after a rain. It's relatively inexpensive, and it's widely available. I think I paid about two dollars a 40# bag the last time I bought some.
Just a thought for consideration...