Adi Shankara’s treatises on the Upanishads, the Bhagavad Gita and the Brahma Sutras are his principal and almost undeniably his own works. Although he mostly adhered to traditional means of commenting on the Brahma Sutra, there are a number of original ideas and arguments. He taught that it was only through direct knowledge of nonduality that one could be enlightened.
Adi Shankara’s opponents accused him of teaching Buddhism in the garb of Hinduism, because his non-dualistic ideals were a bit radical to contemporary Hindu philosophy. However, it may be noted that while the Later Buddhists arrived at a changeless, deathless, absolute truth after their insightful understanding of the unreality of samsara, historically Vedantins never liked this idea. Although Advaita also proposes the theory of Maya, explaining the universe as a “trick of a magician”, Adi Shankara and his followers see this as a consequence of their basic premise that Brahman is real. Their idea of Maya emerges from their belief in the reality of Brahman, rather than the other way around.
Adi Shankara was a peripatetic orthodox Hindu monk who traveled the length and breadth of India. The more enthusiastic followers of the Advaita tradition claim that he was chiefly responsible for “driving the Buddhists away”. Historically the decline of Buddhism in India is known to have taken place long after Adi Shankara or even Kumarila Bhatta (who according to a legend had “driven the Buddhists away” by defeating them in debates), sometime before the Muslim invasion into Afghanistan (earlier Gandhara).
Although today’s followers of Advaita believe Adi Shankara argued against Buddhists in person, a historical source, the Madhaviya Shankara Vijayam, indicates that Adi Shankara sought debates with Mimamsa, Samkhya, Nyaya, Vaisheshika and Yoga scholars as keenly as with any Buddhists. In fact his arguments against the Buddhists are quite mild in the Upanishad Bhashyas, while they border on the acrimonious in the Brahma Sutra Bhashya.
The Vishistadvaita and Dvaita schools believe in an ultimately saguna Brahman. They differ passionately with Advaita, and believe that his nirguna Brahman is not different from the Buddhist Sunyata (wholeness or zeroness) — much to the dismay of the Advaita school. A careful study of the Buddhist Sunyata will show that it is in some ways metaphysically similar as Brahman. Whether Adi Shankara agrees with the Buddhists is not very clear from his commentaries on the Upanishads. His arguments against Buddhism in the Brahma Sutra Bhashyas are more a representation of Vedantic traditional debate with Buddhists than a true representation of his own individual belief