Don't give up! It's possible... I actually play this game alone also, and the only time I get stuck are the bosses that you have to stop from bashing in the cannons.
Personally I always play missions through the tavern, it's harder, but if your having trouble with a boss, go through through(I don't know the name of the other) that hole in the wall .... And the missions are easier, if you're still having trouble, I suggest you work on finding a good set of cards that boost your hp and defense, as well as attack, also try leveling up a weapon with good attributes to at least above +5
Difficulty will depend on which class you play, Assassins are OP but only after you farm end game; until then you are going to suffer and three missions in Chapter 9 (that have you killing 100 mountain, fire and ice giants respectively) are impossible for the class~.
If you have issues try experimenting with other classes~.