I understand you may be frustrated. But this player is just tanking. So, he probably wanted to make sure he was not going to finish in the top 7. And after that was (almost) secured, he came back with correct tactics to try and save some blue packs. When I tanked some season ago I did the same and, I must confess it (pitty on me !) I nevertheless tried to annoy the leader by playing my best team agaisnt him. But at the end, you won. So, no harm done !