The one seedling that was left has not been hit again. I had never seen slug damage on a peony before. I thought that whatever made peonies disagreeable to deer and rabbits affected slugs the same way. Could it be that new seedlings have less of whatever chemical gives the leaves an undesirable taste?

Ha, slugs eat almost everything. Have you had more rain than usual? Maybe they want to expand their gastronomic outlook.

You might try finely crushed eggshells around the base of your plant, that usually cuts the little buggers and jabs them. I've seen them go right through diatomaceous earth no problems. Plus egg shells last when it rains.

I've tried eggshells around hosta before but maybe didn't have enough and they were spread too thin. I stole sand from my husband's horseshoe pits to spread around the survivor. Perhaps that is helping?