If they are airdodging, I like to try and land an usmash, even if it isn't kill percent. It does a lot of damage, hits easily (if you misstime the airdodge slightly you'll often still get a hit) and sets them up directly above greninja, which is a terrible place to be. Bair would be a decent choice for lower risk and lower reward, and if you feel you can win from keeping them in disadvantage a little longer than it is a good choice.
Its nice out of the back hit of utilt if they are DIing away, its the only thing that will hit I believe.
I use it mostly when greninja needs that fast startup aerial, although sometimes that is hard because you are being beaten up and its hard to turn around...
I use it landing a lot as well, to push people back. It can confirm into utilt at high percentages so if they drop shield, that works. or you can time it a little earlier, and the last hit is pretty good range and knockback but NOT a disjoint so be careful vs swords.
And of course, edgeguarding, its great. If someone is going to come up on the inside, don't have a hitbox, and aren't especially susceptible to hydro pump gimp, it can be a nice stagespike. I'd say its worth trying at lower percentages, since you might get a kill and a ledgetech isn't a terrible situation for you. At higher percents, hydro pump is ideal because you can kill with usmash if they go flying.
This might mean bair is a better choice at battlefield, which has both a bigger underside and a platform at the top where a hydropumped person can land to avoid usmash.