koolaid has consistently been top 2 in your region for the past 5 years, it was always relevant to you. I used to have a document of every active player in NE and their known mains and secondaries, and I'll be doing the same when I get into smash 4, it's a good way to track what matchups you should be spending most of your time thinking aboutthis was never relevant before (for me at least).
anyway, starting with what you said
the biggest key is staying in pits face where he can't spam arrows but far enough away where he can't box with you. marth doesn't want to CQC...pretty much anyone. on stage I don't think this is excessively difficult for marth, I'm pretty short on answers for pit's planking though...I had trouble dealing with arrows and punishing in close combat.
also, go into training mode as pit and shoot arrows horizontally with the maximum upward arc you can, memorize that trajectory. when you're in the air, try to occupy that space where he can neither hit you with arrows nor can he dash attack or fair you without you seeing it coming easily. if he starts jumping around while shooting arrows to counter this, you gain easier entry from the ground...for that matter memorize the arc for SH downward arrows too, that hole is almost half of FD