So, I did beat the game as the bard. (If you get to final boss in Dream Quest, it counts it as a victory.) In anticipation of the final battle, I chose the Chorale of Chaos song along the way. (Bards can have a selection of songs that affect battles in some way.) Chorale of Chaos is a tricky one — when you use it, you swap decks with your enemy. I've been burned by it most of the times I've used it. Many monsters have weak decks, and it's their non-card abilities that make them difficult.
I was thinking that would not be the case for the last boss, the Lord of the Dream. But, yeah, it was. His deck has some really strange and powerful cards in it but also a whole lot of stuff you'd see at the beginning of the game. His ability to force you to choose a brutal penalty on you every turn, the number of cards he draws, and the fact that he has a thousand hit points is really what makes him difficult.
I didn't realize that because I'm usually excited by the time I get there. Even though I would have done better with my own deck, I'm glad I at least got to see what he has.