|Alrighty. As per usual, I'm going to start with the story stuff. The story is interesting and keeps the reader's attention. There is good imagery, and the voices are personified well. The ending is cool because it keeps the reader thinking about what is actually going on and might keep them wanting more.
There were a couple of things that kept coming up as far as errors are concerned. Make sure you are separating your descriptors with commas. "Big, green apple." "Shiny, red firetruck." Things like that.
That being said, too many descriptors will cause your readers to trip over content, so you might want to keep an eye on that.
Lastly, and this is more of a suggestion than an error thing, too many adverbs are not a good thing. Slowly comes up a lot. Keeping some in is fine, but you can create conciseness by taking some out. If there is a low-action point, you don't need "slowly." You can also swap out phrases like instead of "He slowly walked."--"He crept." See? Concise but still close to what you were getting at. It's just a suggestion, though. I still use some adverbs in my work, but cutting down on them makes yours sentences less troublesome to get through.
Overall, this is a great start to a budding novel. I am curious to see where it goes and hope you'll continue on.
Below, I have listed little things that you might want to take a second glance at (some of which are covered by things I have already mentioned).
I hope this helps you.
his strong muscular arms- comma between strong and muscular because they both describe the arms
for a while. – Don’t need that.
was three-feet – no hyphen unless you mean three-foot
pointed shaft – I think you mean head. The shaft is wood
slimy green serpent’s head – comma between slimy and green
The shaft pierced – spear, not shaft
four-feet – no hyphen
how old he was and how he – I would change and to or
cliff, and – no comma
fall slowly – Don’t need slowly.
wet thin clothing – Or thin.
he got from hunting – or any of this
backpack, and – no comma
class room – one word. Classroom
his left chest. – Breast instead of chest
Finally, he came – arrived, not came. What you have isn’t incorrect, but the wording has other connotations. Best to be clear.
“Y-O-U A-R-E T-R-A-P-P-E-D H-E-R-E. G-E-T O-U-T.” – The form of this is fine, but it doesn’t make sense. Telling someone that they are trapped AND to get out is weird. I would suggest either “GET OUT NOW” or “YOU’LL NEVER ESCAPE” or something like that. The two statements together is what makes it weird.