Super Auto Pets is a free game I have over 900 hours in, this game is a fun and free "card battler". I do however think this game has a problem with power creep, and overabundance, especially after the most recent expansion.
You can also choose for yourself how casual you want this game to be, with many fun playlists, like a more casual arena, or a challenging ranked.
There is no surprise that this game is the success it is, and i would recommend anyone to try it (it's free).