Touhou New World Boss 28: Youmu
1HELLO DIFFICULTY SPIKE
Difficulty: 9.5/10
You can stagger and parry a lot of Youmus attacks but as we've established in a world of bullet hell bosses fear the bosses that run up to you to beat you up physically. Her screenwide dash attack has one of those awkwardly long windups followed by a near instant attack so you basically have to internalize the attack. Then there's her bombs.
The first one has her creating mini rifts that fire out projectiles at an angle and while this is intimidating it's fairly easy to dodge once you realize you can jump over the rifts.
Then there's her second bomb where she does the obligatory anime sword cut thing that covers half the arena. Sanae had an attack like this as well but that one did lower continuous damage and gave you about a second to react. Youmu on the other hand takes over half your HP andyou have maybe 1/3 of a second to react and dodge. The kicker is this boss is also required so you can't just save her for later you have to get good and win. Thankfully the bosses that follow have some tricks but I've yet to have another boss on par with Youmu or Utsuho in terms of difficulty.