story identification - Human & Alien captured, observed by higher beings

From a collection of short stories I read at least 30 years ago. :/

This is going to be incredibly vague, so apologies in advance.

A human and alien (blob/octopus-like...I think) from warring races are captured by an unidentified higher intelligence and put in an enclosure (more like a zoo habitat rather than a prison cell). They are separated by an invisible barrier and can see each other.

I think they were both trying to escape or trying to figure out how they could kill the other first. There was a bulldozer/loader type vehicle they could use at one point to build a shelter (or something).

I believe they were crafting makeshift weapons as well. There was no communication between the two as far as I remember.

The main tension in the story seemed to be was the race to get to the other first.

This has been stuck in my head forever. I know its a very common theme.

The short stories may have included Asimov.

Answer

This is likely to be Fredric Brown's short story "Arena" (1944). It was previously asked about and answered here.

The alien is not a blob or octopus, but it is spherical in shape and has retractable tentacles that it uses to manipulate the environment.

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.

The alien and the man are chosen by more powerful aliens to settle a war between the respective races. The two are placed in a sandy/scrub environment and separated by an invisible barrier, as you describe.

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.

The barrier prevents them from crossing into each other's zone, but inanimate objects can pass through. They make various missile weapons and try to kill each other. The human eventually "fools" the barrier by rendering himself temporarily unconscious to cross through it, and wins the fight.