It is comfirmed this stacks with Apocalyptic difficulty.
Complete game on any difficulty to unlock Apocalyptic, beat that & get both achievements.
*edit from evil terrorr: Wicked Killington counts as a level, so you need to find all the Trickster keys to unlock the final door to him. (he is always set to Apocalyptic difficulty).
U can check the symbol on each chapter to see which difficulty has been completed.