How do Advaitins justify that Nirguna Brahman is the ultimate reality and that Saguna Brahman is illusory?
He is the self-luminous and formless Purusha, uncreated and existing both within and without. He is devoid of prana, devoid of mind, pure, and higher than the supreme Imperishable (Mundaka Upanisad II. 1. 2).
According to Shankaracharya's commentary, here, "He" refers to the attributeless pure consciousness or Nirguna Brahman.
On the other hand, the word Imperishable (Akshara) means Saguna Brahman (associated with Maya and the cause of creation), in which the Upadhis of names and forms of the manifested universe exists in a seed state. He is declared supreme in a relative sense. Because he is higher than the manifested universe.
Thus the Nirguna Brahman is higher than the Saguna Brahman. So, according to the scripture, Nirguna Brahman is the ultimate reality.
Ref: Upanishads by Swami Nikhilananda.