A lot of it depends on who you're playing, but there are a few things that apply to everyone.
The cannon ball falls at max range and has a separate hitbox, the max range is half of FD at no charge, and just under a full FD at full charge. The cannon does insane shield damage, and can hit your shield twice if you're at the max range, so try to short hop over them whenever possible. The cannon has a long backswing making it punishable if up close, but you have to approach from above, or get behind him, which can be kind of difficult when they start it in the air.
Theres a few ways to pick up mechakoopa which can tip the scales if he's being careless when he throws them out. You can shield to make them fall over, then drop the shield and hit a to grab it. You can throw out a dash attack, and certain aerials to pick them up while they're running around pretty consistently, or dodge. If you get some practice with it, you can pick them up by hitting a just before they grab you, but if you mistime it you'll get grabbed. You can catch it when its thrown the same ways you pick up other items. Also picking it up the first time resets its timer, so it its about to explode on the ground you can grab it safely. If you z drop it it will walk around as your mecha koopa.
His up B is super punishable, but you have to be careful. He can do non directional airdodges out of the cart, and swing his hammer very quickly, so if you try to poke him at the ledge, he can airdodge through it and immediately swing the hammer after which does a large amount of damage. If he drops the car on you with the up b, it can hit while falling, and hits again when it explodes. You can airdodge or shield between the two hits most of the time to avoid the explosion. When he lands on the ground without the cart, he gets locked into an animation for getting it back. you can punish this as long as you avoid the hammer swing he'll probably do just before he touches the ground, which will hit you if you're charging a short range smash attack
The cart can be canceled into a jump which isn't a double jump, its a third jump. The cart also makes him move backwards just before it starts, meaning he can stall in the air and generally just move erratically. If he gets out of the cart without jumping (swervo, or getting hit) he can still cart and jump. His forward air is a big disjointed hitbox, and both his fair and dair have an extra hitbox if he lands while using them. If he catches you in his jab combo, especially with his back to the ledge, you can SDI out of it if he tries to hold the rapid jab (you HAVE TO slam the stick repeatedly, holding it does nothing). his Fsmash is one of his best kill moves and is extremely good at catching people out of readable rolls and getups, not to mention it has minimal backswing, so he can dtilt almost immediately afterwards.
One of his stupidest combos is carting into you, and then jump canceling into up b and swinging the hammer, which works around 60-80%, but will work even lower if you spam jump after getting hit, as you'll jump into the hammer. You can act before he reaches you with the hammer, so do something. A lot of his moves are really good at cutting off options, baiting you into certain actions, and then punishing them, so a lot of it is not being super predictable
