From Cyphus's "Not to Be Taken Orally" Doc Guide:
"Marth
Anytime you have the space, Pill the hell out of Marth. He can easily swipe them away and come closer, but at this point you are still controlling the match. WD-F.Tilt is an invaluable tool against a defensive Marth, if timed perfectly from a SH'd pill since he will generally Jab it away and eat the punishment. Use it. Force Marth to come towards you if you have the room. If you just try to approach him, he will space himself to outreach you, but if you can get him to progress forward, and at the same time you sneak in, you can get inside his range and have the advantage instead. Reverse WD SH pills are your best bet. If Marth goes aerial, dash close (baiting for his F.air) and open your shield. Work as many safe feints and retreats as you can to make your offensive rushes less expected.
You want him to think he will catch you off guard with a dash grab. But stay low, since from above, his UpTilt stops everything you have. If his shield grows weak, your dash attack will trip him, and combo into an UpTilt or F.air at higher damage. The dash also clashes with his D.tilt so it can be a good approach for a ground-defensive Marth, if you follow up immediately with a CCed D.Smash or Jab. Jab puts Marth in the air for a Grab, so mix up single and 2 hits, and dash Grab when he DIs and possibly air dodges backwards hoping to avoid it.
If you can bait him into F.Smashing, shield it and WD out of your shield into a D.Smash/Grab. At low damages D.throw links well into you UpTilt, and UpAir, and possible D.air into Up/B.air/Sex for nice damage. UpThrow is even easier to get a clean SH UpAir at beginning percents. At mid damage its possible to shuffle UpAirs under him but his percent and your distance and speed have to be perfect or he will escape. 40-50% is your best odds at starting the chain. But take note to Sex as well, if you are close and don't think UpAir will come out in time. The slightest difference in speed is worth substituting than risking his escape. At 90% and above, D.throw works well into a F.air KO. Keep in mind to possibly turn around and jump the other way, if he DIs behind you (which is his smartest option).
When you're grabbed, be quick to DI in the direction of his throws to put as much distance from him, since F.Smash connects easily from his F.Throw. If you're thrown up, his UpTilt and UpAirs are going to ****, if you get caught. DJ away if possible and continually DI away from his attacks until you are able to land on the ground. Air dodging into the ground might be your only option if he's shuffling F.airs at the same pace that you're flying. If you shield a close-ranged air attack, but can't shieldgrab because he's moving to the other side, jump out your shield and hit him with B.air or Sex. If you get hit by the F.Smash, you can also DI down and tech roll across the level, but if you're late its usually just better to diagonally DI up and towards him. Even if you are thrown off the level, DI away and recovery from below. If not, an early F.Smash death at 40% is not something to be proud of.
While recovering from the air, pills can be useful in canceling out his F.Smash edgeguard, but they won't protect you from his aerials or Nuetral B. It you can get past his range, feint like you're going for the edge by fastfall and airdodge behind him. Other than that, you are safer going for the edge, from underneath. Hold L soon as you UpB to tech off the side and UpB either- immediately again if he is close enough to be hit, or let yourself fall and try to sweetspot the edge again and repeat. If you are holding Up as you tech, though, you will wall-jump away from the level. If you donnot UpB immediately from this, you will probably not make it back to the stage.
It can be hard to B.air edgeguard him, but faking edgeguard attempts works great since he is floaty. Jump backwards towards him, but then fastfall to avoid a possible hit, then rise back up with your B.air, or F.air him if he airdodged infront of you. If he is recovering from below, simply edgehog and wait to see if he makes it back on top. If so, a rising F.air works well, as with D.throw to F.air if you pulled yourself up as he landed on top since he is vulnerable during his lag. On small, platform levels you are UpTilt and F.Smash target practice, as with the pill being tactically weakened so you will want to stay under Marth, and be ready to jump out your shield with attack in response to his. Doc can actually hold his own very well versus Marth if you play by the book."
I know you're asking for Doc strats, but I like to use Mario vs. Marth. The F-smash range, slightly less lag on aerials, and better recovey IMO help out a lot in this match.
