Stealthily approaching the shinobi from behind, the masked man stops once he comes within mid-range of him and decides to cast them in an illusionary technique. The genjutsu causes them to see an exact replica of himself approaching from their frontal view. With his guard down due to the success of the distraction, he sneaks behind him and places each of his hands on their respective shoulders and whispers a single word into his ear.
Boo.
Now, what do you already know about the illusionary arts ?