How do I deal with Dark Pit players as Samus/D. Samus?
Solution 1:
Problem 1: Spamming
Projectile spam will not help you in this match-up. You need to approach fairly quickly, (unless you are charging your neutral special) which isn't match-up specific, just a Samus thing. What your projectiles could be useful for is approaching - they force your opponent to reflect, shield, or jump. Of course, you should anticipate a reflection with a spot dodge, but not before you've managed to get a little closer to your opponent.
Dealing with Dark Pit's arrows isn't the easiest - parrying each one is probably your best bet, though. You might want to practice this some.
Problem 2 - spacing
Ah, Pit's side-b. The bane of life.
No really, it can be super annoying. Still, zair is a good option, especially when fastfalled. Predicting when Pit will use Side-B is half the battle - it is very punishable.
A good strategy might be to come in carefully with zairs that don't actually hit your opponent, but do keep them at bay, and too close to arrow-spam. Patiently wait until they close that last little gap with an attack, then shield (hopefully parry) and you have your opening.
Problem 3 - reflectors
This one hurts, too. However, you can use it to your advantage. By shooting some obvious projectiles (rockets) as reflector bait, you can sometimes run in and take advantage of the reflector's very long end-lag. Maybe rocket-> down-b roll in -> grab?
Final thoughts
You might also want to have a counter-pick for this match-up, since it is unfavorable.
Solution 2:
I'd play this much more defensively, you've got some very good things going for you.
Your mines can, and should, be used to zone him a bit, to alter your fallspeed etc and/or simply to damage him. When he uses his side-B, you may be able to dodge him (actually not very hard once you know the timing, up-dodge might work the best) and then punish him.
About the grab, you should mix slow rockets with the faster ones, they will cancel each other if one of them is reflected, which let him open for retaliation, and grab. You could also rush him and grab him before the reflect, and it should be very fine.
About his arrows, it's annoying but once you learn the timing you'll be able to "perfect" block it and it's less of an issue afterwards.
Your down-a also has a wide area of effect, if it looks like you're going for a point-blank b-neutral and he tries his reflect or block etc, this attack might work better.