Madhva Acharya (mdhvaacaaryru)(Sanskrit pronunciation: [m@dhva:'tSa:rj@]; CE 1238-1317 ), also known as Purna Prajna and Ananda Teertha, was a Hindu philosopher and the chief proponent of the Dvaita (dualism) school of Vedanta. Madhva called his philosophy Tatva-vaada meaning "the tattva or realist viewpoint". Madhvacarya was born on the west coast of Karnataka state in 13th-century India. As a teenager, he became a Sanyasi (monk) joining Brahma-sampradaya guru Achyutapreksha, of the Ekadandi order.
Views on other schools
Madhvacharya was a fierce critic of competing Vedanta schools, and other schools of Indian philosophies such as Buddhism and Jainism. He wrote up arguments against twenty one ancient and medieval era Indian scholars to help establish the foundations of his own school of thought.  Madhvacharya was fiercest critic of Advaita Vedanta, accusing Shankara and Advaitins for example, as "deceitful demons" teaching Buddhism under the cover of Vedanta. Advaita's nondualism asserts that Atman (soul) and Brahman are blissful and identical, unchanging transcendent Reality, there is interconnected oneness of all souls and Brahman, and there are no pluralities. Madhva in contrast asserts that Atman (soul) and Brahman are different, only Vishnu is the Lord (Brahman), individual souls are also different and depend on Vishnu, and there are pluralities. Madhva criticized Advaita as being a version of Mahayana Buddhism, which he regarded as nihilistic. Of all schools, Madhva focussed his criticism on Advaita most, and he wrote four major texts, including Upadhikhandana and Tattvadyota, primarily dedicated to criticizing Advaita.  Madhvacharya disagreed with aspects of Ramanuja's Vishishtadvaita. Vishishtadvaita school, a realist system of thought like Madhvacharya's Dvaita school, also asserts that Jiva (human souls) and Brahman (as Vishnu) are different, a difference that is never transcended. God Vishnu alone is independent, all other gods and beings are dependent on Him, according to both Madhvacharya and Ramanuja. However, in contrast to Madhvacharya's views, Vishishtadvaita school asserts "qualified non-dualism", that souls share the same essential nature of Brahman, and that there is a universal sameness in the quality and degree of bliss possible for human souls, and every soul can reach the bliss state of God Himself. While the older school of Vishishtadvaita asserted "qualitative monism and quantitative pluralism of souls", states Sharma, Madhvacharya asserted both "qualitative and quantitative pluralism of souls".  Shankara's Advaita school and Ramanuja's Vishishtadvaita school are premised on the assumption that all souls can hope for and achieve the state of blissful liberation; in contrast, Madhvacharya believed that some souls are eternally doomed and damned.  Madhvacharya's style of criticism of other schools of Indian philosophy was part of the ancient and medieval Indian tradition. He was part of the Vedanta school, which emerged in post-Vedic period as the most influential of the six schools of Hindu philosophy, and his targeting of Advaita tradition, states Bryant, reflects it being the most influential of Vedanta schools.

Q: What did Madvacharya believe?
A: Madhvacharya was a fierce critic of competing Vedanta schools, and other schools of Indian philosophies

Q: Why did he oppose them?
A: accusing Shankara and Advaitins for example, as "deceitful demons" teaching Buddhism under the cover of Vedanta.
Did Madhvacharya have any issues with other guru's?
Madhva criticized Advaita as being a version of Mahayana Buddhism, which he regarded as nihilistic.
Madhva criticized Advaita as being a version of Mahayana Buddhism, which he regarded as nihilistic.