See Operation True Heirs for more info.
This achievement will come from finishing all of the missions in Rome. The final mission is "Intercept Marburg at Museum of Art". No matter your choice, this mission will conclude with a fight against Marburg. You will start off on one side, Marburg on the other, and several of his men coming down the staircase in-between. Try to damage Marburg as much as you can, but when his men start getting close to you, take them out. Whenever you take out his men, Marburg will rush towards you, attacking you with CQC. Either run away or keep shooting, Marburg won't attack you for long, but it can be damaging. Once he's done, he will run away, with more of his men coming down to greet you.