Yeah it can be really frustrating with a bad player as a teammate, for example I was playing it earlier and the first time I played I got priest and in the few first turns i got 2 divine spirit and inner fire. I was like ok this gonna be easy. Then the boss exhanges our hands and I was like ok no problem, the combo is still there, and then the other player play inner fire to get 1 MORE ATACK on a random minion. I insta conceded that one.
But also when the other player is good or decent it can be really enjoyable.
It's actually pretty easy to win this brawl, of course if your teammate is playing it for first time and doesn't know about the AOEs of the Boss (at what health or turn he does AOE) and so on, you can lose. I have played 3 games , one with an unknown person, 2 with friends and easily won all of them
I feel you : it took me almost 2 hours to complete it. I finally found someone who had no clue of the win condition BUT was able to adapt. It was overall an excruciating experience. You'll be better grinding 100g by yourself, I wish I had.