I played this game and at the start it is hard but by the time you have 17 cards you mastered the heart of the cards and find the levels less painful than the beginning.
If fact I managed to play 23 levels, 20 wins and 0 losses, how? Easy: the second you realise you are about to be launched into the shadow realm just refresh the page and yes your cards and stats save but not the battle, just hope rng helps you the next game and then you win.