In film, there's a good reason you rarely see your character going up against an insurmountable challenge in the opening act. We need time to build empathy, establish some stakes, grow and learn with them, and then pit them off against any such an arduous foe.
Can you imagine Star Wars opening with Luke or Obi-Wan versus Vader? We're routing for our heroes all the more when we've felt their struggle, and although the "get beaten by a minor foe, recuperate and come back" trope is a staple of many narratives, in gaming it's occasionally the complete opposite.
Primarily because an integral part of the medium is fostering challenge and forcing you to learn the ropes before continuing, a first boss encounter can be a cakewalk as an attempt to heighten your immersion and keep playing... or it can be a brutal reminder that you're Jon Snow, and you truly know nothing.
The following bosses existed to be 50 foot-high roadblocks on the highway of progression. Towering titans laughing at your feeble attempts to make a dint, reinforcing a level of challenge the developers knew would separate the gamers from the 'filthy casuals'.
Hopefully you eventually surmounted them and could continue on your way, but for the vast majority of us, these guys using our bones for kindling was just too much.
9. Cleric Beast - Bloodborne
The entire opening stretch of Bloodborne is intended to shape you as a gamer, and as a fan of creator Hidetaka Miyazaki's work. You're either going to stick with that level of challenge, or you're not. By comparison to every other big-budget title on the market, Bloodborne threatens to damn itself to the aisles of the pre-owned, because even if you beat all the enemies in the opening level, you've then got to deal with the towering Cleric Beast -all on the same checkpoint.
For Dark Souls veterans the guy wasn't much of a threat; simply dodge, duck n' weave through his legs, but for everyone else, Cleric Beast's sheer stature and sweeping moveset is troublesome enough.
This is all alongside a camera that refuses to behave when you're backed into a corner (a trope that's dogged the Souls series since day one), meaning that even when you start factoring in how to take the beast down, the game itself can still stab you in the back.