This trophy is on the first mission and will encounter Mugen on a stand off against 3 gurads by the gate.
Naturally, you'd eliminate 2 enemies in front of yourself (On ground and on the tower) using your shuriken.
Climb the tower, but instead of operating the crane for distraction, use Hayato's stone and toss it between the 3 enemies and the gate.
They'll turn around and Mugen will automatically perform the execution and you'll earn the trophy.