Loosing 100k cups in one day

Some low OVR cards can outperform Prime Icons in-game. In the past, winning depended more on ranking and training so-called "meta cards" with typically Hard/Hard work rates (sometimes training was not needed though), rather than just focusing on OVR and collecting Prime Icons.  The genuine Top-tier players master game mechanics and "exploit" weaknesses in opponents' formations to gain an edge. When I played FM religiously, I consistently made the masters tier and sometimes sneaked into the top 250 ladder. 433A and 433H were the two formations I used back in the day, as you had the most control over the game. Sometimes False 9 worked well with some other good "meta cards". If you lose 2-3 games in a row, take a break—pushing harder will only lead to more mistakes. If you keep losing on Tuesday, maybe you just got a bad luck of facing better players or cheaters that still are in the game, just do your grind another day. Keep in mind every opponent has their style, the faster you realise what they are up to, the faster you can put a stop to it and slap them a couple of surprise goals. 

Fifamobileguide historically had a good list of "beast cards", also pay attention to card reviews.