Just to clarify, you get this trophy after defeating the horde of Sirens at the end of the Desert level who momentarily steal the second horn to summon Cronos and Pandora's Temple and during the subsequent cutscene after you defeat them, not after defeating the three Desert Sirens to open the initial door.
Just clearing that up just in case people wonder why it doesn't pop after the three initial Desert Sirens, as I and other people I've seen have wondered (I hadn't played the game in a while haha).