...she grabs the gun in case that was the invisible monster.

There are a few more suspicious vague echoes from that direction.

(The kid is being very quiet and watchful.)

She whistles a little to get a clearer bounce.

It's not where the rock fell but about ninety degrees to the right, slinking slowly closer.

Whistle. Aim. Whistle. Fire.

It... 

Suddenly turns to run in a scramble of noise when she turns in its direction to aim. The shot is thunderous and releases rather more smoke than she's used to- She can seehear the thud as bullet impacts flesh, and then seehear a pained yowl and the creature running rapidly away. Seems like she might have hit a leg.

Is it leaving a blood trail?

Bit hard to see that at this distance. If she walks up and checks- Yep. Thin, but there.

"Okay. Hopefully it'll go die somewhere, or be too scared to go after us again."

Or maybe it will get more magical and become a terror of the deep desert and one of the reasons the wilds are yet feared even in the modern days of firepower. That'd make a good novel. ...I'm really glad it's probably gone."

"Me too."

"I guess we'd better get back to walking. Or maybe we should think about hiding the magic."

"Yeah, we can keep an eye out for good cache spots."