How do you imagine the battle?

Going by ANH only (for those watched the original Star Wars as a kid, and then read the Mark Hamill quote explaining the showdown at the volcano), Anakin and Darth being two separate entities... How did you imagine the set-up to the duel and the duel itself  ?

I see it as Anakin and Obi-Wan attempting to find Darth Vader for years, and finally, they recieve word from Republic Remnant spies that he (Vader), has traveled to a volcanic world to draw out and murder a Jedi in hiding.

The Jedi duo arrive too late to save the fugitive Jedi, but they are able to confront Darth, who was about to board his shuttle to leave the planet. There is an amazing battle, with a range of volcanos as the backdrop, and a crimson sky caked in smog. 

Obi-Wan is incapitated, leaving Darth and Anakin to take their battle off the docking pad and onto an expansive metal bridge over a smoldering pit. The Force is so concentrated around them, that it has a substantial affect on the surrounding enviroment. A vortex of brimstone singes their bodies, but they continue to clash their blades animalistically. 

Anakin gains the advantage and takes off both of Darth's legs, proving himself the superior swordsman. "I do not wish to destroy you, Darth!" Anakin says to his former ally. "Surrender, and you will be permitted to live."

Darth deactivates his lightsaber and reaches out his hand for Anakin to take, nodding. "As you wish." Suddenly, Darth's hand rises and closes into a fist. He evokes an intense, but short eruption from the volcano below them! The bridge is melted.  The two enemies, consumed wholly in fire, fall to their seeming demises as Obi-Wan watches in horror.

 "I remember very early on asking who my parents were and being told that my father and Obi Wan met Vader on the edge of a volcano and they had a duel. My father and Darth Vader fell into the crater and my father was instantly killed. Vader crawled out horribly scarred, and at that point the Emperor landed and Obi Wan ran into the forest, never to be seen again."  - Mark Hamill

I interpret "forest" as the remote regions of the galaxy. The wilderness.

I never had the distinct luxury of imagining Anakin and Vader as separate characters. I was seven years old when I first saw Star Wars in 1982 and was told that in Empire Vader was revealed as Luke's father. At that age, it didn't occur to me to reject the official, canonical tale.

Therefore, I imagined the battle as taking place above a huge lava pit (though not on a volcanic world) with Vader enraged at Obi-Wan for taking his children from him. He wanted his children to follow in his footsteps and surpass him, whereas Obi-Wan wanted to shelter them from the Dark Side. In my seven year old version, the duel would have started in the very beginning of the movie and come to an end around the time that the movie ended. Sextagenarian George Lucas had just a little more restraint in ROTS than I did before ROTJ.