I see people complaing about different bosses being to strong, but for me the most frustrating boss is actually the 1st one: Linzi Redgrin. There are games that you cannot win against Haro (playing Murlocs or other "swarmy" archetypes for example) and sometimes P.O.G.O. just starts shuffling your Muscles into his deck only to draw them and beat you, but most of the bosses have that - a situation / set of draws in which they are insane. That said that is why you play Heroic - to feel to challenge of facing the more difficult bosses and try to make your deck OP enough to beat them.

Problem with Linzi is that usually, his deck is just better than your starting deck most of the time. Your only chance is swarming him and taking the board from him, which can be really hard since his curve ussually tops out at 2-3 mana and if he starts losing he goes all face. On top of that there are many versions that require different plays against them (against Edwin you should play your Convincing Infiltrator, but against the version with Gang Up! you need to hold it until Gang Up! is played). There have been runs when I wanted to try particular archetype (Kobolds desire, Bomb Warrior), but just wasn't able to get through him like 4-5 times in a row. I think the challenge should come only after you picked something for the deck and have your treasure not before. It is just such a feel bad moment to lose even before you have any chance to customize your deck.

I think he should be at least made into 2nd or more only boss so that you can have at least some treasure/advantage to use against him or his appearance rate should be lowered so that you wouldn't play him 5 times in a row. Especially given that ALL of the other 1st bosses almost can't beat you even if they are super lucky. Wendy, for example, has stolen from me Frost Lich Jaina only to be killed before she played it and played my Wrench-Callibur (basically 16 damage!!!) that also wasn't fast enough for her.