bigsmok3r`s solution works well, but since I do not like those trials up on the hill very much and kept failing from time to time, I ended up using another (solo) farming method for those masamune marbles:
I started up free roam on the sidemission-map "Heisenberg 3". Check if the "Lord of War"-Miniboss is nearby, easiest done with hitting the select-button (whatever that`s called now, I tend to forget that) to bring up a bigger map.
Look around a bit and if you do not find the big red skull in a spot that you can reach in about 10 seconds from your spawn just start free roam again.
As soon as you find a good spawn with the boss nearby bring him down and you`ll get 27 marbles (Insanity 1 difficulty). Pause the game and from there go back to the main menu. Load up the save again and you will spawn at the same spot with the boss nearby, as long as you do not start another mission or free roam. Kill boss, back to main menu, load up, repeat.
The boss spawns three tumors when he goes down to his knees the first time. I used two shotguns (Purgator with poison and Boner with electricity) and the "Arm of Orochi"-Sword with ice, but whatever works for you will do.
Crit chance & damage will help obviously and I enhanced shadow fury as well.
The 27 marbles will drop every time, even without any %-items for more drops. Killing the boss took me 2.5 minutes in the end with all loading times, but that was with level 160 and all relevant skills maxed.
If it takes you to long to kill him you can also try out free roam in mission "Flirty Fishing 1". The boss there I could kill in about 2 minutes (including loading times) with ease, because he doesn`t go to his knees and spawns no helpers.
But the drop rate for the marbles is much lower, about 50% I think. I have not tried other free roams, but it is possible that you can find a better suited boss on a different one.
Useful hint 1: As you level up you get access to higher difficulty levels (up to insanity 7). While it`s not useful to crank it up for the killing process (bosses drop more marbles, but you`ll encounter more immunities on enemies which will be a hassle to deal with), you can easily exploit it for better weapon-upgrades: Before loading your save hit RB, then choose the highest insanity level available. After loading up just go to the forge and start crafting, you`ll get much better upgrades. After you are done with crafting you can just quit out back
to main menu, change the difficulty back down and enjoy better weapons.
Useful hint 2: Probably known by many, but I just dicovered it when I was nearly done: You can cancel out of shadow fury, just press the d-pad down again, you can save some fury with that.