Dr Strange is able to face some of Marvels most powerful Cosmics like IG & Galactus Cyttorak.
Powers-Dr. Strange is the Sorcerer Supreme of Earth; he has unparalleled mastery of the mystic arts, which he uses to defend his reality from otherworldly threats; his primary magical patrons are a group of entities known as "the Vishanti." The Vishanti are a trinity of godly beings comprised of Agamotto, Hoggoth, and Oshtur. Although he has, on rare occasions, called upon the power of the demon Dormammu, he no longer does so. He holds an M.D. in neurosurgery, although his ability to perform such delicate tasks has been compromised due to his accident. Stephen is also an accomplished martial artist.
Dr. Strange's powers are all magical and take several forms. Several main manifestations include:
Personal abilities. As a result of his mystic training, Stephen is capable of a great many abilities such as astral projection, telepathy, hypnotism etc. These are put in a separate category because he seems to perform these functions as a telepath would, not needing to manipulate magic to do it. These abilities can be amplified by mystic energy (as against Moondragon) and/or the Eye of Agamotto (done countless times) to afford Stephen incredibly powerful psychic abilities.
Universal sources. By manipulating the ambient mystical energy of the universe, Dr. Strange can perform a great many functions. Commonly this is seen as mystic bolts, transmutation, telekinesis, standard shields, etc. He can also use this mystical energy to cast spells of a near infinite variety. These spells seem to be quicker to perform but lesser in power than spells using divine energy.
The canon suggests that virtually every human is capable of learning and harnessing magic — considered simply a form of energy in the Marvel universe — through training; however each person has a different potential. The Ancient One saw in Stephen an incredible potential, quite likely the greatest on Earth.
Divine sources. Dr. Strange can channel energy from countless mystical beings in countless dimensions to empower his spells. This can take the form of standard spells ("Crimson Bands of Cytorrak"😉 or just stating what he wants to occur and channeling some being to make it happen. It is unknown at this time what debt, if any, he incurs by invoking the powers that be. There is no quid pro quo, though some entities will feel he is obliged to heed their call when they need help in their conflicts as did the Vishanti.
By sheer force of will "take" the power of another entity. This does not require the use of a spell. He used this ability against Arioch and Shuma-Gorath during the Strange Tales Volume 2 run. The fact that Stephen can do this is a testament to his incredible will-power and strength of mind. This is considered black magic and as such he rarely employs this. Also when taking the powers of celestial entities he absorbs the mind and assumes their duties and roles in the dimension in which they exist. If his will falters, he can lose all sense of self.
Strange's own power is often amplified by the numerous magical artifacts that are in his possession or by artifacts that he uses in the course of his adventures. The two artifacts he carries with him at all times are the Eye of Agamotto and the Cloak of Levitation.
The Eye of Agamotto is a powerful and valued artifact that has many functions. Using the Eye, Strange can see through any lie, deception or illusion, and free others from their own illusions. It is often used to amplify his mind's eye, giving psychic abilities that rival the most powerful of telepaths. It is also often used to play back an area's past events and open dimensional portals. Evil can hardly withstand its light and it is often used as a weapon of last resort.
The Cloak of Levitation allows him to fly without using any magic. It responds to his thoughts as if it were part of him. Dr. Strange has used it many times as a "third set of hands" to attack a foe when his own body has been incapacitated. The cloak is nearly indestructible. It often escapes damage during even the most violent confrontations.
Other artifacts include the Amulet of Agamotto, which functions much like the Eye, the Orb of Agamotto, which he uses daily to monitor the surrounding dimensions for trouble, the Wand of Watoomb, which amplifies his power, and the Book of Vishanti, which contains some of the multiverse's most powerful and secret spells and counter spells. There are countless other artifacts that he owns that can be brought out in times of need.
Note that some other (non-human) sorcerers, such as Dr. Strange's former apprentice/wife Clea, have an additional source of power. Clea naturally generates mystical energy, which she can then use (or, it is assumed, 'lend' to supplicants calling on her). When the Vishanti asked him to fight as a soldier in the War of the Seven Spheres, and refused to allow him to call upon them when he declined, Dr. Strange briefly tapped into this type of energy by immersing himself within the Earth's mystical Gaian aura. However, this energy source was exhausted in his service to the Vishanti during his service in their five thousand year war. His refusal prior to the deal resulted in the Vishanti abandoning him in his role as the Sorcerer Supreme. After doing without the Vishanti's help in protecting Earth, he finally acceded to their demands that he fight for them, but only if they promised to return him to the place in time when he entered their service, and not 5000 years hence at the conclusion of the battle.
For a brief period, research from the Vishanti library led Dr. Strange to tap into catastrophe magic by invoking a syzygy of all the planets, but this source of magic was limited and he ceased using it specifically as a source of his power.
The mansion where Dr. Strange lives, his Sanctum Sanctorum, is located at 177A Bleecker Street, Greenwich Village, New York City, New York, which was the actual address of the apartment building in which the writer of the series lived at the time the story was published. The mansion is almost a magical entity unto itself now that Dr. Strange has imbued it with all sorts of spells mostly protective, but some can be pro-active. It has been used as a way to attack Dr. Strange several times, by Umar most notably.
Doctor Strange's magic has shown the ability to effect even the most powerful of entities, leading many to believe Dr. Strange is Marvel's most powerful superhero.