When I was playing Bobby long ago, being the good reader that he is, he showed me that if you U smash someone as they come down from their second jump, they have no choice but to use a recovery move (most likely won't hit Zelda anyway, and then they'll be free-falling), airdodge, or try to get away from you, all of which, if you know where they're going to go to, can be caught with another U-smash.
My chain is about 10 U-smashes which is quite a lot of damage, iirc, and as it decays it becomes easier to catch them again.
I don't know if anyone does this already, but I find it works remarkably well. You can even fake out another U-smash and get a u-air KO, or LK KO, or even a fresh F smash KO from it.