Your description contains elements of two different stories by Fredric Brown. I think you may be conflating the two stories.
"Arena" is a novelette about a duel between two soldiers, a man and an alien, who are separated by a force field:
There was a barrier there of some sort. It clicked, then, in Carson's mind. That thought projected into his mind by the Entity who had brought them here: "—accident of physical strength will not completely decide the issue. There is a barrier."
A force-field, of course. Not the Netzian Field, known to Earth science, for that glowed and emitted a crackling sound. This one was invisible, silent.
It was a wall that ran from side to side of the inverted hemisphere; Carson didn't have to verify that himself. The Roller was doing that; rolling sideways along the barrier, seeking a break in it that wasn't there.
But there is never any doubt about which combatant is the Earthman and which is the alien:
He shuddered as he looked at the thing. It was alien, utterly alien, horribly different from anything on Earth or any of the life forms found on the other solar planets. Instinctively, somehow, he knew its mind was as alien as its body.
You can read the plot summary at Wikipedia, or you can read the whole story, as originally published in Astounding Science Fiction, June 1944, at the Internet Archive.
"Sentry" is a short short story (less than a page) where the surprise ending is that the viewpoint character is an alien and its repulsive enemy is a human:
He stayed alert, gun ready. Fifty thousand light-years from home, fighting on a strange world and wondering if he'd ever live to see home again.
And then he saw one of them crawling toward him. He drew a bead and fired. The alien made that strange horrible sound they all make, then lay still.
He shuddered at the sound and sight of the alien lying there. One ought to be able to get used to them after a while, but he'd never been able to. Such repulsive creatures they were, with only two arms and two legs, ghastly white skins and no scales.
But there is no force field in this story, which you can read at Project Gutenberg. It was originally published in Galaxy Science Fiction, February 1954, which is available at the Internet Archive.
The two stories have appeared together in several Fredric Brown collections; the ones published in English in 1987 or earlier are And the Gods Laughed and Honeymoon in Hell.