Sauron could be beaten by the god-like Valar (like Tulkas, Manwë, Ulmo), powerful Maiar (like Eonwe, Melian), certain legendary Elves who fought Morgoth (like Fingolfin, Glorfindel), and arguably Tom Bombadil; ultimately, only destroying the One Ring truly defeats him, but powerful beings could overpower his physical form or challenge him directly, as Sauron, a lesser spirit, wasn't invincible, just incredibly strong.
It wasn't through great power that Sauron would be defeated: his strength was too great, and maybe the entire Middle Earth united could beat him, but it was unlikely. Anyone in Middle Earth could have destroyed Sauron, as it would only happen with the destruction of the One Ring, and the destruction was a feat of will.
Sauron is the emphasis in J.R.R. Tolkien's main trilogy, but Morgoth may be the more formidable force. Morgoth was the first to enact a reign of terror upon Middle-earth, with Sauron as his pupil.
At the end of the second age, the High King of the Elves, Gil-galad and Elendil, the King of the Numenorians in Gondor, defeated Sauron in armed combat,  although they both died in the fight. There is a long history of Elves or extremely powerful men defeating Miar - like the Balrogs.
Why was Sauron afraid of Aragorn potentially possessing the Ring? Gandalf explains it in the Return of the King (during the council after the battle of the Pelennor fields) that Sauron would never think somebody would want to destroy the One Ring.
Elrond chose the fate of the Elves and therefore the choice was presented to his children. The reason why Eldarion and his sisters (the children of Aragorn and Arwen) were born mortal was because Arwen chose mortality and therefore that fate was passed on to the rest of their line.
Saruman first appeared during a fourth phase of writing in a rough narrative outline dated August 1940. Intended to account for Gandalf's absence, it describes how a wizard titled "Saramond the White" or "Saramund the Grey", who has fallen under the influence of Sauron, lures Gandalf to his stronghold and traps him.
He will fight in the Last Battle against the Valar and their allies, but will ultimately be slain by Túrin Turambar, the Man he cursed. By finally defeating Morgoth, Túrin will avenge not only himself, but all members of the race of Men.
'The Fellowship of the Ring' (2001)
If Gandalf had stayed dead after his sacrifice in The Fellowship of the Ring, then his demise would be the saddest in the trilogy, but since he came back, the saddest single death scene of all three movies goes to Boromir's.
Yes, Maiar have differing levels of power. Olorin (Gandalf) was at least as powerful as a Balrog and probably more so, but not nearly as strong as Sauron. And the Valar deliberately limited the power of all the Istari by putting them in elderly physical bodies.
Considering how Sauron was during his lifetime, no, Balrogs never strike fear unto him. Sauron has been with Melkor since the Years of the Trees. He and the Balrogs fulfill the most important roles in Melkor's regime. He had no reason to fear them because they were his “allies”.
Why does Gandalf say he is Saruman when he comes back as 'Gandalf The White'? It was just a figure of speech, one might almost say; he was Saruman in the sense that he is now the head of the order or that he would be the good Saruman.
Eru Ilúvatar is the definitive supreme being of all Tolkien's Legendarium.
Balrogs are, of course, stronger in physical power and direct combat, but Sauron was a formidable sorcerer, manipulator, and strategist. Therefore, he would not defeat a Balrog through sheer force, but through his abilities in magic, manipulation, and strategy.
Saruman could've likely mastered the Ring and defeated Sauron, just as it's implied Gandalf may have been able to. It would, of course, have transformed him into Sauron in all but name though.
The Posttraumatic Stress Disorder of Frodo Baggins
J.R.R. Tolkien in The Lord of the Rings accurately portrayed the signs and symptoms of what is currently labeled Post Traumatic Stress Disorder. Frodo's condition logically follows his experiences of less than a year in the War of the Ring.
Merry and Pippin both died decades BEFORE Aragorn died. Merry and Pippin died in FA 63, while Aragorn died in FA 120 - and it was after this that Legolas and Gimli sailed to the Undying Lands. All of this is made patently clear in Tolkien's own timelines.
We die the first time when our breath leaves our body. We die the second time when our loved ones return our body to the ground. And the third death, and final death, is a moment, sometime in the future, when our name is spoken for the last time.
But at the Council of Elrond, Gandalf very specifically says he fought all nine. He drew four of them off when he fled northward at dawn; "This helped, a little, for there were only five, not nine, when your camp was attacked."
Sauron needed Gandalf out of the picture permanently, as he was the only enemy with the knowledge, leadership and determination to thwart him. His biggest mistake was ego. He succeeded by trickery, by his ability to deceive others. He conquered so much land that his ability to control became out of reach .
Biography. Very little is known of Smaug before he attacked, in TA 2770, the Lonely Mountain and the town of Dale. What information exists dictates that he was created by Morgoth at some point after Nírnaeth Arnoediad for use in his fleet of winged Fire-drakes.