There is something wrong in the French version of the game :
In the screen where you select the arena mode, it presents the classical version and the special version which is "Imitation" where you play with the deck of your opponent. This is normally the "Mimic" mode.
So I thought that I had to draft the worst possible deck.
What a mistake !
I sent a message to the support about this problem.
They gave me this solution, which is working for me :
"Please try using Verify / repair function in GOG Galaxy.
Run GOG Galaxy.
Click Gwent on the installed games' list on the left side.
Press MORE button, select "Manage Installation" and then "Verify / repair".
Wait. The repair function can take time depending on the number of corrupt files it has to repair. "
Easy 9-0 for me, most games, including the last finished with 1 card up. I played Francesca and didn't encounter any mirrors, fought against 7 Woodlands spirits though.
Key cards seem Phoenix, Muzzle and Gaunter, which are either strong or counter each other. Other than that Ihuarraquax is pretty nice if your highest is Old Speartip and also punishes the greedy Zoltan.