Find a tall wooden wall with an opening, or a tree branch. Get guards to chase you there. Get on top, face towards a guard, have the "Rope" selected, hold Triangle to throw, and move the left stick back to jump down and hang the guard. Repeat for 10 guards.
Progress doesn't save unless you reach a checkpoint in a mission, or something else that triggers it. So if you are in trouble, run away and recover. Don't quit or die before getting the trophy, otherwise your progress since the game saved last will be lost.