Well, i nearly done 1k battles and still nothing. I tried clan battles, gyms, computer controlled players, even training accounts to grind but here i am with 0 plates. And i battled with a guy who has 4 forms of arceus(fighting); shiny mystic shadow and normal. So i think there is a weird odd for players idk, there are already bunch of arceus(fighting) and some players own like 4 at the same time while others don't have one.
Edit: After sending this, i tried to battle with players on maps. 3 or 4 battles and i got fist plate. I'm not sure it's a coincidence or not but give it a try