Just to elaborate on how I feel about it.
In a singleplayer game, there is no "fairness", for me. It's just you and the computer, and it should all come down to how much you can endure until you stop.
Because there is no other human opponent, you have all the time to consider what you are going to do, and hence bring out your best against what the computer throws at you. Hence, the game can never afford to be predictable. It must always reach some higher standard which to you as a player must feel like is beyond your reach.
Often, RPGs will have such fights, but they will be made optional. Supposedly the makers of the game don't want that fight getting in the way of you completing the game. And yet all the same, I, as a player, want to experience the game fully, so it's not like I am going to let go of the optional fights anyways.
Which is why I liked Betrayal At Krondor. It always threw fights at you which were ridiculously and monstrously hard, and I loved it. It would range from a dozen moredhel with poisoned quarrels to nighthawks that would respawn within the combat round to a handful of witches who manage to keep casting Unfortunate Flux at you to the endless ghosts at Sethanon who can batter your party down quite quickly.
And yet, with long days of patience, reloading, changing your plans, keeping your characters in the best shape, even they were defeatable. The first priority of any good RPG is to have a dynamic character creation/development system, which allows you the potential to waste those enemies anyway.
Thing is that even in Wizardry 6 Expert difficulty, the bossfights are designed to be impossible and unbeatable, and yet a player with enough perseverance and understanding of the game can defeat them.
There is never supposed to be a limit to difficulty in a singleplayer game. Ever.