As far as I'm concerned if you can use a debugger it automatically shouldn't qualify as the most difficult ever.
As per the compute shader post from a few days ago, currently I'm "debugging" some pretty advanced code that's being ported to a shader, and the only way to do it is by creating an array of e.g. ints and inserting values into it in both the original and the shader code to see where they diverge. Its not the most difficult but its quite time consuming.