Luigi is really annoying, I can see why Spikes thinks its in Luigi's favor. I can't say its Doc's 3rd hardest matchup or w;e you think it is, but it's definitely tough. Let's get some discussion going about the matchup.
His combo game on Doc is actually quite good, BS like Dair->Nair -> moveset, plus with his amazing WD/better range then Doc force Doc to play super gay and spammy. It's also relatively safe for him to edgeguard Doc with a WD off stage/drop down Fair/Bair, ledgehop dair or nair-> finisher at lower %s. Plus its really risky to attempt to combo him cause of his low hitstun and nair combo breaker, so you essentially have to rely on edgeguards/spamming pills and pokes to keep him out.
Essentially from my perspective its a more annoying version of Jiggs with much slower aerial mobility. And also, (my 2 cents here) Doc v. Floaties is super lame and boring to play, Doc v. FFers/Marth/Sheik is fun, but he loses most of those matchups. Why do I use this character again?
Dair>nair? Wtf?
Okay
Let me straighten some things out:
vs Luigi
His WD is long, but it's NOT GOOD
It forces him to commit to movement. Remember how you can't attack for part of a wavedash? Same thing applies to him. So what you do is either sh and stick a nair there for him to run in to, or cc dsmash. It's not even a 50/50. If you see him sliding and you're above 50%, sh nair. Otherwise, CC dsmash, or just clean dsmash.
Use pills to force the spacing for these situations.
Comboing him is weird, but Doc can actually do extended combos on Luigi. Your first 50% or so vs him is gonna be 50/50 guessing games, though.
jab>dsmash is a godsend when you catch him not CCing stuff. This is the first 50/50. Either they CC and jab/dsmash back (which you can CC and punish with... DSMASH! Doc is stupid), or they take it and fly off.
EDIT: SEEMS I WAS TIRED AND FORGOT SOMETHING!
A good Luigi will DI the jab upwards and nair, assuming you hitco into dsmash like I do. Against this, you wanna shieldgrab. It's really hard to read, though, because of how fast he pops out nair and how similar it looks to him just wanting to eat jab>dsmash. Be careful.
END EDIT!
Your next 50/50 is dthrow. If you expect them to try and CC anything, just grab him. After the dthrow, I'd recommend utilt, since it beats the ever-precious nair pretty cleanly, and, if spaced right (not hard to do vs luigi), pops up perfectly for a combo. Also, usmash beats all his aerials.
The 50/50 comes from whether he wants to try and nair again after being hit by utilt/usmash (they're pretty much guaranteed at lower percents). He can either attack again, or try to retreat.
Given his VERY slow aerial mobility, most Luigi players will choose to try and steamroll through. You'll prolly see a lot more nair, but some may get desperate and try dair. What you need to do is keep tapping him from below. Tipper pretty much everything. Get too deep, and you're gonna get naired. Tag him with the end of moves as much as possible, to reduce recovery time.
Follow his DI, and uair the hell out of him. Doc's like cf in that, if he tippers Luigi with uair, Luigi can't really do much about it aside from DI. End combos with nair, maybe bair or a smash, if you magically get them low.
Comboing Luigi is different. He's so slow in the air that the game becomes "follow under him and tipper with launchers repeatedly". If you suspect a nair and you're too low to reset, just shieldgrab that ****. It DOES have disadvantages.
Also, if you KNOW he's going to nair at higher percents, back off and give him breathing room. Let the move come out, and then smack him upside the head with fair. Pretty great stuff.
And all this is WITHOUT Doc's (imo) great edge game.
Seriously, watch for Green Missle. You cape one of those, and he's probably dead. Cape a misfire, and you're not only guaranteed a stock (unless he misfires again. D: ), but will find yourself getting some kind of badge or something for it. Personally, when I smell misfire, I just go to where he's gonna land, and wait. Misfire>HUGE LANDING LAG xx DAMAAAAAAAGE!
Bair can stop him pretty easily, too.
Really, the only thing Luigi's recovery has is the ability to spit out a billion aerials (most of which you can actually outspace with fair/bair) and misfire. Unfortunately, in CO, Luigi's misfire seems to be a 1/3 chance. Sometimes a 1/1. Our Luigi player combos into misfire pretty reliably. I dunno if he's the luckiest guy ever, or what, but it hurts my feelings when he misfires to make a comeback. =(
The thing with this match is that you have to play INCREDIBLY safe, and INCREDIBLY gay.
Do NOT commit to anything, unless you're pretty much guaranteed to not be punished. Luigi WILL come sliding in, and you WILL eat a combo that makes you lose the CC ability that I think makes this match so bearable.
Seriously, that CC is a godsend. When you kill Luigi and you're sittin' at less than 100%, you should normally be able to get 40% wracked up on him before you die. Once you have that lead, it becomes a MASSIVE uphill battle for Luigi (a lot of characters, actually).
What you end up doing is trading hits willingly. CC any ground moves, and retaliate. By the time he can break your CC, he'll be at kill percents, and you can start the cycle again. Just watch out for grabs, and keep your defense on point. If he gets a combo, it pretty much resets this cycle of gay, so keep him hitting light, punish him for anything that could net him anything.
Beat grab by sh nair
dtilt/jab with cc dsmash
dsmash with cc dsmash
ftilt/dsmash/fsmash his usmash but don't CC it
usmash/utilt/shieldgrab nair
sh nair anything else that moves
and jab his fireballs, then start your own pill set-ups
These are the basics of fighting him
Oh, and ftilt footsies, but those are a little more advanced, and harder to explain. Pretty much, you kinda want to zone Luigi. On stages like BF, try to stay under the corner of platforms. I like to keep my back to the edge of the stage, with Doc underneath the outside corner of whatever platform. You can FH pill to get him if he comes from above (change the timing for tight pill bounce pattern lulz), sh the pill under the platform to cut down his approach options from below, or standing pill. Make SURE you do empty SHs sometimes, so that they're always guessing about how to dodge/stop your bs.
Jab is good, too. Has HUGE range, and good priority. Beats Luigi before he beats you, most the time.
I made it sound like Doc ***** Luigi, but I still feel like it's slightly in Luigi's favor. If you refuse to play REALLY gay zoning games/do bizarre combos, then Luigi can and will walk all over Doc. You can't give this stupid guy any leeway, or else you'll find yourself sittin' pretty on a fresh 60% damage, compliments of your local gay plumber. Or you'll be dead.
That and how gay misfire is. You have NO idea how happy it makes me, seeing that move get knee'd.
Maybe I should just nair...
People be sleeping on Luigi. He's really good.
EDIT: ALMOST FORGOT!
di BEHIND Luigi when he's comboing you. It makes it a LOT harder (or impossible) for him to follow up. I didn't start doing this until I smoked our Luigi up and he told me it, but it seriously cramps Luigi's style. He can do combos still, but they don't hurt as much, and rarely end with you being ****ed off the stage. Also, you can play DI games by DIing away from him later, and botching up his backward DI reset (not that he has many).
Do it, seriously.
THE MORE YOU KNOW!
EDIT:
Keep in mind, I
just woke up. Most my info should be on point, but there may be an instance or two of me not thinking clearly.
I do hate fighting Luigi.
It's frame perfect and so fast you can't really "count" the frames, so you have to just get used to the motion. My personal preference is to go from straight up to down+right... the direction you're facing is irrelevant. Eventually you'll get used to the timing 
