Link: spam projectiles and roll, as well as up smash and dair on landing
Boomerang only hits you when it's thrown. When it comes back, you can ignore it. Arrows leave Link open. If you can get to him just before he shoots his arrows, then you get a free hit. Bombs are really the only threat, because he can pull out two of them, and they have some sort of HP on them. If you're getting hit with up smashes, up tilts, or up aerials, it's because he's already forced you to approach from the air. Bait those attacks and punish him. Down air is predictable, so just don't challenge it unless you're on his left or right, and not below him.
Bayonetta: Down tilt into trademark combo, with tons of priority
Learn to SDI. If Bayonetta is doing her Witch Twist, SDI upward and away the moment she does it. Unless you're using a fast faller or heavy character, this shouldn't be too much of a problem. She won't be able to continue her combo without using her second jump if you SDI the Witch Twist. If you're caught in the Witch Twist, SDI down and away instead. Against her, you have to be pretty patient. If she's coming in from the side to start her combo, just block both the slide, and the hit that follows after. She can still try to grab you if you shield her a lot, though, so keep that in mind.
Corrin: Nuetral B and side B, as shielding is not an effective way to punish
Neutral B is predictable. If you see Corrin doing this, you can either do nothing, or approach from above and hit with a down air. Dragon Lunge, I think it's called, is annoying, but if you have a good out of shield option, then you can counter it once he lunges the spear-like attack into the ground. But you'll have to be somewhat close to him to punish him. Otherwise, trade hits with him when he flings himself into you after the lunge.
Cloud: Crazy range with pretty much all his aerials
This is a surprise to me considering Corrin's neutral air is just as annoying as Clouds. The difference here is that Cloud's only circles once, whereas Corrin's spins all around his body starting from both his front and back. The obvious aerials are his up air and down air. Don't try to challenge his up air, obviously, unless you have some sort of attack that can go through it first. Down air, same as Link. Avoid being underneath him when he does it, and punish him from the left or right in the air. Or shield and grab. Forward air looks like it has some good lag behind it. Airdodge into an aerial yourself while moving into him. Not sure what to say about his back air. His down tilt can leave him open if you shield it.
Link is a rare character to fight, so I think he's got the advantage there. The other characters are just really good, so overall, don't feel bad for losing against these characters.