Furthermore, there's an inordinate amount of grinding to get the upgraded cards, and 90% of the upgraded cards are strictly better. On many occasions, upgraded cards are the difference between a mediocre deck and a very powerful one. Fortunately, a mediocre deck doesn't really matter, since the AI is piss-poor. There's a reason the higher-level AI had powers a human player doesn't, such as upgraded health or marks - they would be absolutely decimated otherwise! The card history does not always stick around long enough for new players to know what in God's name is happening. The sound effects are bland, and there is no music aside from the intro screen, making for an entirely dull experience. There are far better card games on the market, with many of them being just as free as this game. Please, play one of them instead.
This game is simply not good. Matches are completely mindless - each deck revolves around one strategy and mostly ignores the opponent, unless they need to destroy a permanent or something. There is an irritating amount of luck, far more than just card draws, and a bad roll could have dire consequences - for instance, the shield that can freeze attacking minions, and freeze lasts for AGES. With all your minions constantly on the attack, it's impossible to have any semblance of strategy in regards to, possibly, not attacking. Effects are unclear as to when they change - for instance, if some of my light-emitting minions were destroyed, Hope (the light shield) would sometimes change defense at the end of my turn, and sometimes at the start of my next turn, with me having no idea as to the rhyme or reason of it. Criticism is continued in my next comment.
Protip for high scores: combos multiply the base score of the enemy by the number of the combo, thus rising exponentially, and only reset when you fire a shot. So, let explosive enemies (bazookas and rocket helicopters) blow up allies and themselves, and save the most expensive foes for last.