Vritra performed penance for one of the longest tenures, ie sixty thousand years.. therefore he posessed immense ascetic power.. In addition to this, Brahma given him power + all kinds of illusions..
Indra was empowered by the full might of all Devs while he faced Vritra in battle.
O Dhananjaya, all celestial weapons arewith Indra. The celestials, from fear of Vritra, imparted at the time all their might to Sakra. Gathered together in one place, thou wilt obtain allweapons. Go thou unto Sakra, he will give thee all his weapons. Taking the bow set thou out this very day in order to behold Purandara."
But still was defeated and knocked out by the serpent demon. Vasisht revived Indra and empowers the lord of gods with his own might. In addition to this, Shiva gives a portion of his power to Indra and weakened Vritra with his Shiva - Jvara (which is embodiment of Virabhadra).
'While Vasava of great energy was thus being restored to consciousness by the high-souled Vasishtha, his strength became greatly enhanced. The illustrious chastiser of Paka then, relying upon his intelligence, had recourse to high Yoga and with its aid dispelled these illusions of Vritra. Then Vrihaspati, the son of Angiras, and those foremost of Rishis possessed of great prosperity, beholding the prowess of Vritra, repaired to Mahadeva, and impelled by the desire of benefiting the three worlds, urged him to destroy the great Asura. The energy of that illustrious lord of the universe thereupon assumed the character of a fierce fever and penetrated the body of Vritra the lord of Asuras.
In addition to all these amps, Vishnu himself posessed Indra's Vajra.
The illustrious and divine Vishnu, adored of all the worlds, bent upon protecting the universe, entered the thunderbolt of Indra. Then Vrihaspati of great intelligence and Vasishthaof exceeding energy, and all the other foremost of Rishis, repairing to Him of a hundred sacrifices, viz., the boon-giving Vasava, the adored of all theworlds, addressed him, saying, 'Slay Vritra, O puissant one, without delay!'"
Maheswara said, 'Yonder, O Sakra, stands the great Vritra, accompanied by a great force. He is the soul of the universe, capable of going everywhere,endued with large powers of illusion, and possessed of great celebrity. This foremost of Asuras is, therefore, incapable of being vanquished by even the three worlds united together. Aided by Yoga, do thou slay him, O chief of the deities. Do not disregard him. For full sixty thousand years, O chief of the celestials, Vritra practised the severest penances for obtaining strength.Brahman gave him the boons he had solicited, viz., the greatness that belongs to Yogins, large powers of illusion, excess of might, and superabundant energy. I impart to thee my energy, O Vasava! The Danava has now lost his coolness. Do thou, therefore, slay him now with thy thunderbolt.
The full might of all Devs + Vishnu + Shiva and Vasishta’s energy empowering Indra + Shiva - Jvara weakening Vritra was needed to being this demon down.
So he's undoubtedly most powerful snake or serpent in all epics or Puranas.