Hi,
Breaking down gunshots is what I always do if possible. Body, sub, mechanical, tail, shells etc. I've used mostly Wwise as a middleware and it's easy to get multiple different sounds when every layer has as at least 3-6 different sounds and they go to their own random container. And then you can randomize the timings, pitch and high-pass filter of every layer.
I think it's doable in Unity as well with the same principles in mind :)
Maybe monster sounds could be done with different layers also but it needs a bit more careful approach. Maybe you could have at least two layers: the main monster sound and something to blend under it. Both can then have multiple variations. Of course depending on the monster you can have more layers like scream, gargle, growl, breathing and a main definitive monster layer.