AC:Valhalla has some severe issues with combat. The biggest one is related to how stamina works for NPCs. I will explain it in three steps:
(1) What I would intuitively expect from a pseudo-realistic stamina implementation in video games,
(2) the way it currently IS implemented into the game,
(3) the way it SHOULD be implemented into the game.

(1) Stamina should have one major purpose: Define a state for your character that affects what he is able to do at a given time. If you are out of stamina, then you are out of breath, severely reducing your capability of performing tiring actions like lifting a weapon or shield, sprinting, dodgeing, etc.

(2) There are two stamina systems in the game: The one for Eivor and the one for NPC enemies, especially bosses.

(2a) If the player runs out of stamina, Eivor cannot dodge nor parry anymore. Regarding the stuff written above this is a good implementation because it elevates the game above simplistic "hack'n'slash-click-spam" as the player has to think about his actions. Stamina is a regenerating and actions cost stamina which is even affected by the weight-stat on your equipped gear.

(2b) For bosses and enemies stamina is a non-regenerative stat (it's not even called stamina, but "stun"), displayed by one or more yellow bars above their health, that can be taken away by parrying their attacks, hiting them with heavy attacks or by destroying "weak spots" on their bodies.

• If the "stun" bar falls to 0, the enemy is stuned, opening a window for a special hard hitting blow that takes away a high amount of their health.
• This is a one time opportunity, if the stun bar is gone, it's gone for the entire duration of the fight.
• Stagger mechanics seem to be related to that aswell, at least for zealots:
  • If you parry their attack combos while they have some stamina left, they sometimes stagger.
  • But if their stamina bars are depleted, they are unimpressed by your further parries. Shield zealots can not be staggered anymore leaving almost no chance to hit them with melee attacks. I don't know whether this is a bug or intended, but as soon as their stamina is gone, they get super overpowered by removing all stagger opportunities from them.
• Contrary to Eivor, stamina/stun never affects what a NPC boss can do. Even at 0 stamina/stun, he will continue his hard hiting attacks, dodge, block, etc.

=> Not only is this absolutely [censored] taking into consideration what stamina actually is (1), it also takes away every feeling of reward for your good timings.

(3) This is what a proper implementation of stamina would look like:

• Enemies should regenerate their stamina just as Eivor does. If you deplete it, it should have ramifications tied specifically to the type of enemy you are facing: No more rune attacks (red hard hits), shield zealots could let their shield fall, murderers (dagger enemies) could be left without the ability to dodge, enemies at 0 stamina could always be staggered when being parried, etc. etc.
• This way would make gameplay rewarding as you have to play aggressively to outrun an enemy's stamina regeneration in order to stagger him and open opportunities for counter attacking - a paradise for tuning difficulty settings
• Stamina/stun bars could be tied to an enemy's weak spots: By destroying them, some bars are simply taken away, making it more easy to drain his stamina to zero and get more staggers to land hits on heavily defensive enemies.

I don't think these suggestions are anything new to the world of gaming. I lack any understanding how the current unintuitive and unrewarding design could have been developed and could have been rated fun.