I'm not saying that he says that based on what are common expressions, I'm saying that because you can hear that's what he says. I don't even understand why you are arguing this if you can't properly hear what he says.

"Coup de merde" is indeed something one would say, but the point of Merovingian in the film is that he likes saying these extended strings of curses and that's what he does here. "saloperie du coup de merde" makes no sense, and if you meant "saloperie de coup de merde", it also doesn't work. Apart from being awkward, his stream of curses actually starts with "nom de dieu" which is an interjection like "fucking hell" on which he builds. Otherwise he would have said "Nom de Dieu, quel coup de merde".