if/when you do meet the shades again, would you ask them what they know of marind and her disaperance/death? they might know something about it, marind wants us to help her after all
it would seem to me if you wanted to defeat the shades it would require a great deception, so great that you even decieve yourself as to what your purpose is, if they are a mirror, they would see the deception and mirror for instance weakness or the want to go deeper into necrovion, they would mirror this being weak themselves or allowing you deeper, and when they are weak whilst still believing yourself weak you would attack them, continuing the weak thought, obviously surprising yourself when you win, or something along those lines