Personally, I go to the retopo tools readily. They’re a quite effective modelling tool once you’re adept with them.
So, I personally would probably use extract to quickly pull out a big poly blob referenced to the body properly, quickly run over the “Pack” with the inflat brush with backface masking turned on to give it some body, and then retopo it into some decent geometry.
Now, for the hi tech rifle the same character was using, I’d probably go outside Zbrush. Not that you can’t do it in Zbrush, but Im just more comfortable with that sort of object in another application.
Zbrush allows you the flexibility to work as you wish.