É provável que este seja o conto de Fredric Brown " Arena " (1944). Anteriormente, foi questionado e respondeu aqui .
O alienígena não é uma bolha ou polvo, mas é de forma esférica e tem tentáculos retráteis que usa para manipular o ambiente.
It seemed to have no legs or arms that he could see, no features. It rolled across the blue sand with the fluid quickness of a drop of mercury...
It seemed to be studying him, although for the life of him, Carson couldn't find evidence of external sense organs on the thing. Nothing that looked like eyes or ears, or even a mouth. There was though, he saw now, a series of grooves -- perhaps a dozen of them altogether, and he saw two tentacles suddenly push out from two of the grooves and dip into the sand as though testing its consistency. Tentacles about an inch in diameter and perhaps a foot and a half long.
O estrangeiro e o homem são escolhidos por alienígenas mais poderosos para resolver uma guerra entre as respectivas raças. Os dois são colocados em um ambiente de areia / areia e separados por uma barreira invisível, como você descreve.
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.
A barreira impede que eles se cruzem na zona um do outro, mas objetos inanimados podem passar. Eles fazem várias armas de mísseis e tentam matar uns aos outros. O humano eventualmente "engana" a barreira tornando-se temporariamente inconsciente para atravessá-la e vence a luta.