This should have been obvious to anyone who has played the recent tavern brawl, but I guess it should be mentioned since it doesn’t seem that it was obvious to the developers at Blizzard during its testing process.
Blackrock/Naxxramas style adventure bosses simply just do not work with the dungeon run type of PVE content.
They were designed to be defeated in a specific way, which is absolutely fine when you have the luxury of fine-tuning your deck for every such encounter, but this kind of design is ill-suited for the random nature of roguelike dungeon runs.
The most glaring examples of this issue are bosses Garr and Vaelastrasz which can hard-counter entire deck types.
To defeat Garr, you pretty much have to out-tempo him with high-value, low-cost minions in the early game.
While Vaelastrasz favors a hand-dumping approach, but can also counter strategies that revolve around card generation and strategies that revolve around key cards (such as treasures).
The issue with this is that, while beatable, it’s a clumsy design that severely limits the viable options of play, which is highly antithetical to the structure of the dungeon run and ultimately affects the tavern brawl’s replayability.
And since Blizzard intended for the brawl to be replayable, as evident by them launching a community challenge, it would probably be a good idea for them to design such content with actual replayability in mind.