It's still an issue, in my opinion. I think once you reach a wave, there should be an option to practice it independently of the others, and then once you've figured out a strategy, then you can give the whole thing a shot again.
It was incredibly frustrating on Challenge 4 and 6 to be able to just cruise my way up until the final wave, only to then die almost instantly. I would have to then spend an additional 8 to 12 minutes just getting back up to Wave 5 every time, something I could do with ease, only to then last maybe a second longer and die again. If it takes 10 minutes to get to the final wave, and I need fifteen attempts to beat it, that's two and a half hours of shit: I'm just repeating the waves I know how to beat, and are no longer challenging, again and again just to even attempt the final part. Sucks.