One of the techniques I've found is the conversion options out of sweetspot d-air.
Sweetspot d-air occurs at the very start up of the move and the spike hitbox causes vertical knockback on a grounded opponent,
I started experimenting a lot with this almost a month ago after seeing all the crazy Japanese Greninja gifs and found that it is actually a very reliable kill set up. It has worked consistently to the point where I land it all the time. In this explanation, I will abbreviate SweetSpot to "SP" for convenience. Keep in mind that this combo is guaranteed ONLY if you can land the d-air without them teching.
Combos:
SP D-air ---> falling u-air spike -----> f/d/u-smash
Percents: Works best 100-120% with no rage
Side notes: You can actually rack up damage with this before landing the kill; because of the slow fall from u-air spike, you are actually able to SP d-air them as soon as they land to repeat the u-air spike.
SP d-air -----> SP d-air---- > U-air spike ----> F/d/u-smash
Percents: 90%-120%
Side notes: Slightly more precise timing than the above combo. Easier to escape and better for racking damage than landing the kill.
SP D-air ----> shadow sneak:
Kill Percents: 110-140%
SP-dair -----> F-air/N-air (n-air doesn't kill reliably)
Kill percents: 130-140%
-Situations for landing SP d-air
Punishes:
Most people would not see it as an oos option, but it is very viable for countering the more laggy moves in a characters moveset. Short hop SP d-air out of shield is a great option for starting this combo. It's also a reliable move for reading rolls and spot dodges, as well as punishing a whiffed move.
Set ups: For whatever reason, players tend to air dodge to the ground when you use Greninja's d-throw around kill percents. They do the same thing with Greninja's d-tilt and these reactions are likely in response to the m trying to predict your u-smash or another kill move. Take advantage of this; when you d-throw or d-tilt, run where they land and d-air them during their air dodge landing lag. This is the set up that has been most reliable for landing the sweetspot d-air most often for me.
Will post link to video examples in a few
edit: Link to youtube video examples:
https://www.youtube.com/watch?v=i0cs4dh8CS8
I apologize for my bad, iPhone camera quality. This was tested in traning mode, so take note of that.
Some of this tech is just me showing examples, and have been discovered already