This can be done as early as getting your horse in Claiborne, but I'd recommend waiting a while longer so you're strong enough (I did it at level 25).
After getting your horse go back to the railway station near Jolly Roger. Nearby you should see a crevice and on the other a side a field of bushes. You need to get a run up and jump over the crevice on your horse. Now go up the winding path and scan at the end of that area to find a very small village with a single NPC.
Here she'll give you a side quest. Now go back to where you jumped the crevice and scan around that area. You should discover a new dungeon. Make sure to save before entering.
The dungeon is linear just follow the path. The regular enemies shouldn't pose much issue, yet. Once you reach a single treasure chest, open it. You'll now fight a boss. He doesn't do anything special, just beat him down and heal when required. Now you'll have a cursed ring that reduces all your HP to 1 and you can no longer heal.
You now have to survive navigating out of the dungeon. Again it's pretty linear with a few box/switch puzzles. The light guardian comes in handy here, especially on a character with high MTC as the summon ability will wipe out all enemies (put it on Virginia). Once you find the exit return to the NPC at the small village you got the quest from.
You can now plant berries, among other things. Use up one of your heal berries to plant in the soil and the trophy will pop. Also this is a great way to get tonnes of berries so don't worry about using them up to plant them!