“Run! Run! It’s in the building!”
Our feet scrape and thunder as we stampede like frightened hens down the long corridor toward the backroom seeking shelter. Deep-seated fear ices our veins, for some insane reason a crazed grizzly has smashed through the large paned window, its wretched howl echoes throughout the building. Behind us, along the pale green walls of the hallway, its thrusting shadow seems immense as it grows larger and longer huffing its way toward us.

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“Run! Run! It’s in the building!”

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There is a hole in this opening after this point. It dulls your opening line. A Grizzly Bear just crashed into the house. What was that like? What happens in the next paragraph is what the people are doing after that event. Why leave out the breaking glass, growling angry bear, furniture breaking and getting tossed aside?

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Our feet scrape and thunder

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On what? Wood floor? Linoleum tile? Pea gravel? Straw bedding? Say what there feet are scraping on will help me hear the thundering they are making.

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as we stampede like frightened hens down the long corridor toward the backroom seeking shelter.

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I liked this line, a lot.

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Deep-seated fear ices our veins,

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A little to melodramitic. Makes it sound cliche.

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for some insane reason a crazed grizzly has smashed through the large paned window, its wretched howl echoes throughout the building.

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Telling. Cut for some insane reason. It is something that would apply to a person that did somethign irrational, not about the motivations of a wild animal. Things should be happening to fast for such a contemplation from your MC.
The restcould be axed IF you start with the bear crashing into the house. This is a line that lets the reader catch up with events. have the events unfold before the readers eyes.

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Behind us, along the pale green walls of the hallway, its thrusting shadow seems immense as it grows larger and longer huffing its way toward us.

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The MC would have to be looking behind him to see this. He should be running for his life.

Deep-seated fear ices our veins I think this reads better if you cut "Deep-seated"

some insane reason a crazed grizzly The use of both "insane" and "crazed" made this read awkwardly, I don't think you need both in there, just one would be fine.

its thrusting shadow seems immense as it grows larger and longer huffing its way toward us. I think the "seems" weakens this sentence--I'd prefer an "is" there instead. And I think you need a stronger word than "huffing"; to me it just sounds like a soft word and doesn't really convey the danger and tension you need there.