Well, not exactly a card game as much as a deck builder game, "Slay the Spire" can be both easy and brutally hard at the same time. It is still in open access/development, but available on Steam. I would go watch some YouTube footage as there have been plenty of people posting streams from it. It is not really multiplayer (yet), and is more of a survival type dungeon run. Again, easy to play, but can be hard (with some almost impossible bad luck type fights), however can be fun to see if you can survive.