You would say the most silent in the world. Then he could hope when the deed was done to slip quickly from the window, to wade the moat, and to get away at his leisure. That's understandable.
你一定会说他要的是世界上声音最小的武器。这样他才能指望事成以后，很快就穿过窗户，蹚过护城河，从容不平地逃跑。这是完全可以理解的。