Slugs don't touch my shiso. But we don't have banana slugs here, only the small ones.

The seeds need light to germinate, as well as the constant moisture that all seeds need. So it's notoriously difficult to start.

But once you've established shiso in your garden, let a plant or two bolt each year. Don't try to save the seeds, just let the plant mature and die in place. You'll have volunteers forever.