I used to sort of have luigi as a secondary, and this is some of my limited (and potentially not 100% correct) knowledge:
- I dislike Yoshi's because a lot of things simply push you onto the ledge. (as luigi)
-As krazykyzo said, laser camping can be annoying (for luigi)
-Depending how good a luigi is, chances are he'll approach with some combination of d-smash, f-tilt or maybe turnaround up-tilt (which isn't quite as good)
-D-smash is safe on shield cause of low traction (luigi's), try jumping over and drilling.
-Also his recovery has very few options, if you learn how it works you can edgeguard him well.
Hope I could help!
I used to play luigi a bit, here's how I felt
-Yoshi's is an annoying stage 'cause so much stuff pushes you off, you have very little room to move around
-As krazyzyko said, laser camping and be annoying for luigi
-PS or YS are probably your best bets because of the reasons I said for YS earlier and because you can camp a bit on PS and you have a low ceiling.
-Chances are luigi will approach with a combination of wavedashes and d-smash or f-tilt
-d-smash safe on you shield 'cause of low traction, try jumping over it and drilling
Hope I could be of some help!
Sorry
nevermind