That's just it. Running through the area again isn't a challenge, it's simply rote repetition that has to be gotten out of the way before you can pursue the next thing you haven't actually figured out yet (beating the boss). Honestly, if the levels were randomized it would make a lot more sense because then that would be something new to navigate. But no, it's a pointless two minute slog through a bunch of been there, done that. There is literally nothing the player can do to change the state of the game during that time. It's superfluous to the current objective, so why does it exist?
This kind of thing does make more sense earlier in the game, when a new player could use extra chances to practice the combat system. But later in the game it's simply a drag, something to be trudged through as quickly as possible that might as well be performed by a robot. This is the kind of things interactive media gives us the opportunity to eliminate.
And what's more, frustration IS a bad thing. Frustration with the game, that is. Frustration at yourself for being unable to overcome a challenge is your fault, but when the game wastes your time, that's frustrating and it's never a good thing.