Well I disagree. I got my first fist plate within 10 battles however it took me 120 battles to get my second one. If you claim to have done 600+ battles and not have received a fist plate, then you certainly have bad luck.
As Auke said if they were easier the arceus form would lose it's value. If the palte were to be received after a certain amount of battles then all the fun would be taken out of it and would make it another version of the previous arceus events which required you to grind on SQs .
If you've lost all hope for this event , then I suppose you could wait until Patrick releases them through mystery boxes (Considering there are really few of em by the end of the event and Patrick decides to "fix the trade and auction economy". )