When facing sword characters in neutral a bait and punish style works the best. A lot of sword characters' aerials can be punished on whiff or if they're poorly spaced on your shield. Mostly, try not to rush in all of the time. This is especially crucial against Marcina. Their kit is mainly used to poke and box out over-aggressive opponents. Use your mobility to bait out commitments and punish that. Inkling also is faster than most of the sword characters (especially in the air) so Inkling can use its mobility to control the tempo. The only exceptions are Roy/Chrom and Cloud. Roy/Chrom are faster in the air and on the ground and Cloud is faster on the ground. Inkling also does a lot more damage than most the sword characters, with the exception of maybe Ike if he lands a Nair, so you don't have to win neutral as many times as they do. Shulk is a special case though. Rushdown works better against him since his moves are so slow and he wants his opponents to keep their distance.
Parries are also great since sword characters depend on falling aerials quite a lot. I find parrying works best against Ike and Cloud. Ike's aerials are slow and he tends to use them as he's falling. Clouds aerials are all pretty safe when they're spaced and he tends to do them a lot to pressure an opponent's shield. Also, be ready to spot dodge tomahawks since they tend to shift to that when they realize their opponent can parry them. I don't really parry the other sword characters very consistently but I'm working on it.
Inkling's kill throw is also an asset in most sword MUs. Marcina and Roy/Chrom have kill throws but their's won't kill until 185 or so, and if you're surviving that long you're probably already winning. Pit has a kill throw but Pit is underwhelming anyway.
Finally, you should have an advantage in edgeguarding versus most sword characters. Inkling can be edgeguarded but sword characters have to be very precise since they don't always have lingering aerials like Wario's down air or Mewtwo's Nair. You shouldn't get too complacent, though. Marcina can go much deeper than the other characters since their up special goes high, so be aware of that. Cloud's Dair can linger but he can't stay off stage very long since his recovery is pretty bad. Ike can only Fair you if you approach the ledge carelessly. But, he can also go for a two frame Dair or Eruption so make sure to mix up your timing. Roy and Chrom can only Fair you if you get too close to the ledge so going deep against them is preferred. When edgeguarding them it's best to attempt this (for the most part) when they have high damage (~80+). At high damage, they have to drift close to the ledge to recover and they can't mix up their recovery as well. Ike is the only exception since he has quick draw. Also, both Chrom and Ike's up special recoveries have no hitboxes above or behind them when they're spinning, so as long as you get above the hitbox in front of them (and it is very big) a Dair or a Nair will take care of that stock.