Sorry, don't think there is such a thing as sword fly. Do you mean sawfly? Its probably unlikely to be one of these either and more likely to be the caterpillar of a moth called the Cutworm or as you say, slugs or snails. Look out for any slime trails and use slug pellets under a tile next to any plants. Have a look at night too with a torch and see if you can spot either caterpillars (which hide in the soil during day) or snails or slugs.

I find slugs and snails ignore slug pellets to some extent - they prefer petunia when they can get it! My money's on them as the culprits. As Fractal suggests, go out with a torch & you'll find them.