Just downloaded this game after leaving it for a while. That means all my data got wiped, im fine with that. My point is that i only have basic start cards available to me right now and i destroyed all four bosses relatively easily (the last boss is indeed very difficult). There is a grindwall if you want to unlock more elaborate cards, but every boss is beatable from your very first game. You just need to know the combos and how the cards work which does take some run throughs.