Do the gunsmith optional mission (and all yellow optional missions).
If a side-op is yellow, that means it’s a top priority. The gunsmith side-op isn’t yellow, but you should do it anyway!
If a boss is too tough, try hiding.
There are some bullshit bosses in this video game. One fight in particular is some seeeeerious bullshit. If you get there, and you find that you’re not making much progress, try hiding out, calling up Quiet, and letting her take care of things. You’ll see what I mean when you get there.