The idea is that you would lock yourself into a difficulty with some special settings.
All missions must be played at the selected difficulty setting or higher, and, once available, all missions must be played with their challenge active. To make it even harder, it could be required that all missions are beaten the same amount of times before they can be repeated (if you beat 1.1 and 1.2 twice, hardcore would prevent you from playing them again until 2.1 and 2.2 are also beaten twice). This would prevent you from farming the easiest level for resources.
Of course, this can already be done as a self-enforced challenge.
All missions must be played at the selected difficulty setting or higher, and, once available, all missions must be played with their challenge active. To make it even harder, it could be required that all missions are beaten the same amount of times before they can be repeated (if you beat 1.1 and 1.2 twice, hardcore would prevent you from playing them again until 2.1 and 2.2 are also beaten twice). This would prevent you from farming the easiest level for resources.
Of course, this can already be done as a self-enforced challenge.