Hello again pkblaze! So you're managing to beat Marth using Snake now? Hmm... but you want more help against Marth using Kirby now? IMO, I'd probably stick with using Snake against a Marth as the match-up apparently is 70:30 in Marth's favor, but I'll try help you nonetheless. Okay where to start...
Okay to start things off, IMO, I think Kirby should be playing defensively in this match-up. Keep an eye out for any openings and punish him for it. That should be the general mindset for the Kirby player. Although, be aware if he intends to miss attacks on purpose because he can trick you and counter you if you're not quick enough to punish him/you think you'll be able to punish him, but he recovers and counters you while you have just initiated your attack. Therefore, I think you should punish him with grabs just to be safe. Kirby's grabs are still useful in this match-up as they can lead into combos which are always good. Although then again, be aware about one of Marth's fatal grab combo's: When you're at 0% and he grabs you: expect him to use the foward throw and a TIPPERED F-Smash immediately after he throws you. (He can sneak in 2 pummels if he wants, but I think they should use only 1 depending on how good your mashing skills are.) If he successfully tippers you, you'll eat 25% damage which is quite a lot of damage. 27%-29% depending on how many pummels he got on you. So avoid early grabs. Once you're around 20% damage, this combo won't work. Also, any mistake you make Marth will punish you with his >B as it does a good amount of damage and has fairly good knock back to it so keep an eye out for that too as a lot of Marth's like to use their >B. If he doesn't use it to punish you, you could just sheildgrab grab him out of it, but it's hard to do since Marth takes a HUGE step foward when he does the attack.
Next point, Marth outranges Kirby BADLY in the air AND on the ground. Sadly, one sucky thing about this match-up is that Marth can just stand there and be campy with his sword because he can almost swat back any attack you use on him. Therefore, I think you're limited to using Kirby's Bair approach to actually get in there and attack Marth. Also, Kirby's only safe move for this match is his ^B because it is one of his moves that has a fair bit of range to it. However, it is fairly predictable so don't use it too much. Of course, Marth can play offensively in this match-up too because he outranges Kirby. Marth's main form of approach is short hop Fairs. I'd recommend powersheilding or avoiding and punish him once he lands. For Marth's grounded approach, they like to use D-tilt. This move is good for him because it's fast, fairly hard to punish, a good poker and they can use it to inch foward if they want as well. However, this approach is easier to avoid because you can just jump over it and hit him from the air. These are 2 moves you should look out for when a Marth is approaching you.
Next for the duration of the match, Kirby will want to stay grounded against Marth. Sadly, Marth is not too willing to fight on ground and wants to keep the fight mostly in the air because of his Godly air game. Don't try to take him on in the air because like I said before, Marth outranges Kirby in the air. You'll just get mauled by his Fair's and get juggled around like crazy if you try and take him on. If the Marth player is good at getting people off the stage using his Fair, you might eat a tippered Dair and will get meteor smashed off the stage in the process. Marth's also like to try this on people who are recovering too. Luckily for you however, this shouldn't happen to you often because of Kirby's good recovery and it is quite hard for good Marth players to accomplish. Since Marth players like to attack from the air, shield whatever he trys to do to you and punish with a grab. Also, I think I may have already mentioned this, but what you're aiming to do is to bait Marth's attacks and punish him for it.
Probably 1 thing Kirby has over Marth is that Kirby can edgegaurd Marth better than what Marth can do to Kirby. Once Marth is knocked off the stage, edgegaurd with your Bair's. Don't give him a chance to return! His recovery is also fairly easy to punish as well. If he doesn't grab the ledge during his ^B, you can punish badly for it. 2 last things you should know about as well: If Marth shields one of your attacks beware of his ^B move out of shield. This move is excellent for retaliation as it is quick which will make it hard for you to block it, has good knockback to it and it has invincibility frames at the start meaning that you can't interrupt him once he starts the attack. Secondly I wouldn't leave your shield out for long either because if Marth manages to hit you with his neutral B move: the shield braker, it'll eat up a LOT of your shield and may break it depending on how large your shield was to begin with. If he breaks your shield, expect to eat a tippered F-Smash or a fully charged shield breaker if your friend isn't good at spacing it properly.
Lastly, neutral stages you want to go against Marth on are Final Destination (To limit his aerial game due to no platforms being there.) or Yoshi's Island Brawl. (IMO, one of Marth's worst neutral stages. Because it's slightly harder for Marth to gimp people here because of the support ghost and that 1 huge platform can get in his way because it tilts.) Umm... probably other stages you want to take Marth to are Skyworld (It's easier to gimp Marth here.) and probably Jungle Japes. (Because it'll be harder for Marth to get Ceiling KO's with his Uair, U-Smash and U-tilt.) I probably wouldn't take Marth to Battle Field or Norfair (My favorite Marth stages BTW.
) because this will only compliment his air game and Marth has the potential to maul people on top of platforms especially with his U-Smash.
That's all the info I can give you for now and hopefully some of it will help you out in your next battle against Marth using Kirby. Maybe other people can help add to this? I feel like I am missing something...