Yeah, I couldn't complete the hardcore levels. Frankly the two before that were murderously difficult, although I did get those in the end. As best as I can figure out, you need to use walljumps to be able to move forward while facing backwards to slow down the nightmare wall, but I couldn't find any way to do it fast enough on the hardcore level (and it was very hard on the second-to-last regular level). Am I missing a mechanic? Because the nightmare wall really bumrushes you on those last levels.
I don't think the controls are clunky for the most part, although it is very easy to get into the wall-slide mode when just trying jump over a block. That level that has you jump between two blocks at the end is very annoying because of it.
The music distorting when the nightmare gets near is cool, the rain of fire too but it's hard to see because the screen also goes black.