If you want DA BIG GUN (Ozutsu) there are few ways to get it: Fight a lot of lords in the Saikai lands (though good luck with that, its rare), or upgrade samurais in that region to Ozutsu troops, then promote them and steal their stuff (you can just kick them out of the party afterwards). I think it should be possible to also get it from enlisting to a lord there, upgrading to that position and then leaving in the right time.

Some tip is for Enlistment: Probably the best place to get enlisted in is the Tokai lands. You can upgrade to Hatamoto troops, which have the best equipment in the game. On top of that, you will get paid ♥♥♥♥ ton of money.
Also, if you fight yourself wanting to save some of the equipment (since you lose everything if your lord gets defeated), you can buy a workshop in a town your lord visits often and store your precious stuff there.

Thats all for now. Feel free to add some pro tips of your own.