Androgens, GH, quality skin firming lotion applied 2-3x daily. After that it’s a crap shoot. I’ve seen lots of guys competing in masters shows who are peeled inside out but still have a little loose skin in the lower middle abs likely from losing a lot of weight later in life. Skin elasticity is genetically determined like most things.
Not sure if masteron would be something to avoid in this respect. Some users report drying of the skin / looking like an alligator bag etc. The opposite of what we’d want to firm skin that’s lost some elasticity.
One option, and I’m not suggesting this but it’s there, is to run your compounds to obtain some level of water retention. I’m sure you’ve seen the guys who have defined abs over a slightly bloated belly. I’m not taking about a huge gut. Just not a vacuumed men’s physique type waist. Anything that makes that skin more taught is going to involve some subcutaneous fluid retention