Using coarse sand as a slug deterrent will save most of your leafy greens from damage by pests like this one.
Sand (especially coarse sand) works better than anything else I’ve tried in my garden as a slug deterrent. Just sprinkle the sand around the plants you want to protect. I don’t like using wood ashes because they can change soil chemistry — plus, many people may not have wood ashes available. And I find beer traps to be awful to clean up.