Sure, lets talk about the movies only here.
This is Saruman during his first (and only) on-screen fight:
Gandalf is giving him trouble. Yes, he wins the fight in the end, but hes very much also thrown around, and hes bloodied and bruised by the end. Hes not shown as invincible and unfightable.
Then, this is Saruman failing to stop Gandalf, the most valuable and powerful prisoner he couldve had, from escaping:
Thats a big setback for him. In fact, its this setback that later leads to his and Saurons defeat.
So no. Saruman is not portrayed as infallible and only winning in the first movie. Not even close.
No, it emphasizes that despite his considerable power, Saruman was not infallible and experienced setbacks throughout both of the first 2 movies. But you wouldnt know that, since it seems you either havent watched the movies in some time, or you didnt pay attention at all.
Ok?
...you havent actually read any of the books, have you? Aragorn never fought Sauron in the books. Sauron didnt actually fight ANYONE during the Third Age.
The movies originally had Aragorn fight Sauron at the end, but they cut that out and replaced it with the troll, bc again, Sauron didnt fight anyone in the books, and they wanted to keep it that way.