you'd just have to time it right, i dont bother using dair's against flyers/floaters because they can di or "flap" their way outta of it, unless ive read the opponent long enough and actually go to where they will go and dair them.
against pit well, in the beginning of battle most of them well start spamming the arrow so i usually jump and hit them with the laser (aiming downwards) and approach them that way though ofcourse i trade dmg still because of the speed those arrows fly.
keep up the WoP's because that how i usually get my kills.
when hes approaching from the air (and your on the ground) and you know that hes going to do an aerial at you just sidestep and immediately dsmash, you can pull that out pretty fast, but it may take a few to get him, to do with it. but he'll be popped up, i suggest a preemptive rising nair if you know he likes to comeback down with an attack, im pretty sure he will be the one to get hit (afterall his range isnt a marths anyway).
but you should try focusing on mindgames and reading ur opponent, its hard to find a fool-proof combo in brawl nowadays (and with rob)
if hes at the edge, and he tends to roll, pretend to dash at him and when he tries to roll behind you, punish him with a reversed ssmash, or just a dsmash.
the downB i find helps me alot, but you cant let it go to ur head because Pit can easily reflect it, dont be afraid to use it tho, he may not be expecting it. if you do attempt using his gyro, may i suggest using that throw glitch in which you dash right after you throw?
because at low percents + throw glitch and immediately after dash you connect a ssmash, now that leaves you feeling all warm inside :D
also if he likes to hang around the ledge, try running off the ledge real quick and immediatly do a bair that will push him against the ledge and if he cant tech fast enough, its most likely a kill. try watching a few ROB vids (with good ROB players ofcourse). i think tommy's? yeah his videos showed an attack like that.
im sorry if some of my advice may get a little obscure at times, just ask which parts you need clarification on heh. hope it helps D:
if only my problems were with a pit, its the canceled-out-of-shield-fair-with-DI-away-from-my-shieldgrab-marths that really annoy me.
BOOOO.