I never used the word "Assassination."
It's all about deception and lightning-fast movement. The sound of a thousand birds suddenly rushing into your ears? The sudden, erratic flashes of light pulsing all around you? That's enough to confuse and terrify any victim. And before they have any idea what's going on, they have an arm through their heart.
Rasengan's unbeatable in close-quarter combat. The characters, however, are supposed to be ninja, not gladiators.