Best option for slugs and snails is ducks, so see if you can encourage any that may be in the area. Failing which, patrol every night for few weeks and you'll break the back of the problem - you will still need regular patrols thereafter but usually less frequently. I squish mine but others have less violent means of removing them.
Re buttercups - there is no good news, I'm afraid. You have to weed them out, getting as much of the root as possible, then stay on top of the weeding. Buttercups also mean your soil is too wet and claggy; try to work on the soil structure and drainage. One of my beds is either solid as concrete in summer, or wet and sticky in winter - so I've been digging in compost which has helped a bit. I'll grow a cover crop in it after summer and dig that in, which should help too.