First the tags are removed, so your layers will be named /Onepiece_Torso
. Starting the name with a slash will ignore any folder tags. You don't need a slash between tags. You can use nothing or a space. Also I suggest putting the tags at the end: Onepiece_Torso [skin:Onepiece_Torso][merge]
. That way you don't have to read past the tags to see the name.
I think your problem is you have many layers for the same skin. The same name is what you want, so they go in the same skin placeholder, but they need to be in different skins, eg the layers:
Onepiece_Torso [skin:skin1][merge]
Onepiece_Torso [skin:skin2][merge]
Onepiece_Torso [skin:skin3][merge]
That will results in the files:
skin1/Onepiece_Torso.png
skin2/Onepiece_Torso.png
skin3/Onepiece_Torso.png
In Spine there will be a skin placeholder named Onepiece_Torso
and the region attachment for it will depend on which of the 3 skins is active.