Protect is a must.
Protect will guaruntee a +1 Speed Boost, as well as scout for potential Rock, Fire or Electric attacks.
Other than Protect, since I assume you aren't using Ninjask for Baton Passing, I would suggest either Toxic, U-Turn or Endure. Toxic lets you cripple an opponent if his demise is imminent. U-Turn lets you escape to live another day, but you'll lose the +6 Attack. Endure is situational, but if it gives you a pseudo-Focus Sash if you really need it.
Okay, here's a set for All-Out Attacking Ninjask, not what I'd recommend but it's what you want.
X-Scissor /Aerial Ace / Night Slash / Dig
This is, literally, the farthest you can get coverage-wise without going into Special Attacks. Ninjask also gets Metal Claw, but it's insanely weak and Night Slash/Dig is probably better.
This moveset hits
303 Pokémon neutrally.
475 Pokémon super effectively.
0 Pokémon not-very-effectively.
0 Pokémon with no effect.
That's the best I can do. If you're desperate for coverage, you could dip into his Special movepool.
Giga Drain, Hidden Power, Solar Beam, and Shadow Ball.
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