This is my take on what a Darkness themed Silvally GX deck would look like, I'm building this for my local meta so here's a list of what I hope to beat with this deck:
Listed in order of priority
Buzzwole/Lycanroc
Zoroark/Lycanroc
Gardevoir
Volcanion
Greninja
Decidueye
DM Necrozma/Magnezone...