I am definitely concerned about this. If battles like this are too hard, it may ruin the game for me. I don't want the game to be so difficult that it's not fun anymore. I don't want to have to practice for days just to figure things out. The game has a lot of elements that are already looking as though they might be a challenge, and difficult bosses that kill you in three hits will not help matters.
They just might be creating a Zelda game that will only appeal to a niche group of hardcore gamers, if they're not careful to balance the difficulty with less punishing segments. I know that some people want to feel like they're getting "more game for their money" and want the game to last a long time, but I don't feel that making the game difficult/impossible for average people to beat is the right way to do it.
It's hard to make these determinations from demos, though, and I think we'll need to wait until reviews are posted on major gaming sites to get an idea of what the game is really like. I'll give the game the benefit of the doubt until I've played it myself. If it's impossible to beat, I'll be sure to whine about it on this site... a lot. I will warn you, rest assured.