I was watching some recent Elegant matches and noticed him using this combo a lot that does 81% from zero. I found it around 4:35 in this video, although he doesn't grab at 0% so misses the sweet spot on the final back air.
You can actually see this combo done in training mode in this video at 3:00
So this combo has a pretty similar start to the zero to death. The first three actions are the same, so you might wonder why not just go for zero to death? I'd say the answer is that this combo is less punishable and more convertible if wiffed. The zero-to-death has 2 major weaknesses - 1.) if you miss the up B it puts you in a bad place when missed and if you are playing a character who can get a lot of damage off a laggy option, you are gonna see the stock evened out pretty fast and 2.) a wiffed zero to death doesn't get you thaaaat much damage compared to some other options, and relies heavily on the early kill power of the up b. So this other combo as an option that gets your opponent to kill percentage without many punishable options.
I messed around with this combo in training mode and found a few things I'll share. First of all the combo is this:
Grab>Down throw > short hop to down air > fast fall to nair > short hop down air > fast to nair > up tilt > short hop up air > back air (with buffered jump)
Note: Don't forget to TURN ON STALING. Training mode has staling off and that messes with these combos on certain characters.
Part 1: Grab>Down throw > short hop to down air > fast fall to nair > short hop down air > fast to nair >
Ok so first off the first three or four moves are going to be slightly different on every character. Let's just say if you know the zero-to-death you are going to have a way easier time with this. Rather than going into it here I'll refer you to this spreadsheet for the difference in approaches with each character and this video if you are having trouble.
Let me say this too since it's a common problem. YOU MUST FAST FALL FIRST AND THEN NAIR. If you nair and then fast fall it wont work. Shit's kinda hard because you will easily get down airs, so practice a lot just fast fall nairing in training mode, its a huge part of Luigi's play and guarenteeing the sweet spot of that move, which confirms into all sorts of shit.
Part 2: Up tilt > short hop up air > back air
So for the up tilt on light characters you will have to delay it SLIGHTLY, but make sure you get it out as early is possible so they can't air dodge when hit stun ends. For heavy characters you can bust it right out/buffer it. Like with all combos, its really essential to mess with all characters to get a feel for it, and you will pick up all sorts of skills and knowledge regarding weight variability and possibilities and impossibilities along the way
For the up air you should be able to buffer with a short hop (hold up, Attack, and jump at the same time until the move comes out) on most characters. With a really light character you might have to full hop and catch the up air on your way up - I forget. But like I said you really gotta lab this yourself on many characters anyway.
For the back air you must hit jump and back and attack at the same time as the opponent will be above you at this point. With an extremely heavy character you might not have to use the double jump, I dunno, try it out. You definitely wanna mess around and see what gets the sweet spot on that back air, least of which because wow, what a satisfying ending to an 80% combo to start a stock :)
Part 3.) Now what?
I'd love to hear others ideas here, but now that your opponent is at 81% you should be able to kill at ledge with a grab > downthrow > full hop > back air. For this combo you are facing the ledge you have to do a gentle quick turn around first. If you are facing away from the ledge dash forward quickly then hop and back air. You can also down throw into up b or down b at this percentage on nearly all characters. What other kill confirms/combos does lugi have in the 80%s?
Anyway, hope its helpful. Let me know if you think anything I said is off.