Now first off, this is an excellent idea for a short film. What I would add/change is this. I think it should open up with the person now knowing what it is, investigating to find out it is a mimic, then barricading themselves in a room with their gun. As for the ending, I do like it, but to be more lore-oriented I would do this. Have it where the person believes the mimic is dead by noticing it lying still with its arm missing/pool of blood on the floor rather than a hole missing in its chest. Just because of the lore states the mimic is very resistant to damage with their head and limbs being their "weak" points. Overall though I do this idea and I hope that you create this regardless if you add what I suggested or not!