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human
[Last Airbender] Is fire bending inherently more powerful than the other elements?

I'm watching Avatar: The Last Airbender for the first time, and I'm halfway through Book 2. For those that haven't watched the series yet, **there are spoilers below** (I don't know how to do the spoiler tag in this forum). 

So Aang is beast at Air, and he's getting pretty good at water and just learning earth. However, as he believes - and his previous incarnation, Roku - that he must master all four elements in order to defeat the Fire Lord (who only knows the one element). Even the intro acknowledges this.

Also, the Fire empire has conquered most of the world.

Why does the Avatar need to know all 4 elements to beat the Fire Lord (and this is *before* the comet arrives, meaning the Fire Lord is more powerful than the Avatar while knowing only the one element)? Why is the Fire empire so powerful? Is fire simply the strongest element in existence? Why can't an expert air bender like Aang beat an expert fire bender like the Fire Lord?

Admittedly, I'm not on Book 3 yet so maybe I'm still missing pieces to the puzzle.
gpt
The Avatar has to master all 4 elements either way, it's in the job description.  Fire Lord Ozai is a particularly powerful bender.  While Aang is rather skilled at air bending, he is still just a little kid, and needs every advantage he can get to beat the most second most powerful fire bender in the world.