Something of interest, perhaps:
Everyone here knows about stage builder, yes? Well, each grid block's height is close to, if not exactly, the height of Luigi's second jump. Keeping that in mind: a few days ago, I tried my hand at the spike.
The way I practiced a bit of Luigi's D-air spike was basically by chucking opponents off the edge in training mode and in vs. mode and watching their recoveries. Given conscious recognition of the length of Luigi's second jump and some prediction of the CPUs' trajectories and/or DI after they were sent on their trips half-way to the blastzone, I decided that I should try a certain method of spiking them. Run off the edge, second jump, and then D-air(spike).
Given that I was jumping up towards my opponents, I made as conscious an effort as possible to attempt to spike with the upper part of Luigi's body ( \ <-- this side, the side tilted upwards), to avoid getting caught within an unfriendly aerial just as I jumped.
Also, this appears, to me at least, be a pretty good gimp if one can manage to combo the opponent from 0% to the edge and F-throw them. Moderate damage from the combo should give the throw some semi-decent range (if not damaged highly enough, attempt to set up a B-throw I guess), but not place the opponent excessively far away from the edge. Running off of the edge and either fastfalling before second jumping or waiting before second jumping or second jumping immediately (depending on how high up or far away the opponent is from Luigi) can lead into some interesting spikes.
((If someone has already done this in match-ups that are up on Youtube, I'd love to see them - there's no way this idea is original
))
Spiking people who wait too long on the edge as mentioned by Hippie (or Kigbariom? idk...) is definitely something I have to start trying...
OH OH OH! And - if one makes a stage in stage builder with just a few jump-through platforms anchored in empty space, one can practice the second jump->D-air(spike)'s spacing. It isn't necessary to have an opponent to do it for the first couple of tries. By dropping down and second jumping and D-airing in training mode with the camera zoomed in, one can see Luigi's hands spinning. If that whoosh of air (I don't have any other word for it) is barely peeking over the edge, the spacing of the second jump relative to the platform was done correctly...
^_^ Just a few things I felt I should share.
EDIT:
Oh oh - and because you're jumping up as the opponent is coming towards you at an angle rather than chasing the person from above with a fast-fall (assuming they're trying to recover after the throw, of course [which they should otherwise they may taste a second jump->U-air]), characters larger than luigi *should* be spikeable (sp?)... But I haven't tried that yet...
