It sounds like you're having trouble with players countering during your strings, in that case, just be aware of where all the gaps in your strings are and be willing to give up the hit to scout it out. Whenever you're in a situation where hitting them is the obvious choice, walk try waiting instead. Pick up on habits, notice when they like to counter, is it during recovery or after whiffing attacks? Are they countering when landing? If they are, are they using the double counter, where they counter once then immediately counter again right after?
Personally, I enjoy fighting characters with counters, because once you have counter scouted, it becomes incredibly easy to land big punishes. My favorite answer to a counter, especially players who spam it, is to just charge my Fsmash until the counter frames are gone, then let it rip. Sometimes I even hit the counter intentionally a few times with small stuff like jabs just to let them think it's working, then I'll just Fsmash them out of it later on to take a stock. No point in shutting down a habit right away if it'll help you win later right? Now if they are really good with their timing for counters and it's completely unpredictable then you can just give up a few more hits and try to grab a little more often to keep them guessing while scouting for the moments when they seem most likely to counter. This way, while you may not get the chance to punish the counter, you can get a better idea of when your opportunities to land your K.O attacks are.
Basically, since counters are a huge commitment, if you are always making them guess then you're less likely to see it, but if you know they are willing to use it, you can pretend to go for that obvious punish attempt to bait it out. You can also try playing a character that uses counter for a while and become proficient at using it. Then it becomes easier to scout out those moments when a character would want to use counter, but you could also just play the match up a bit until you start to notice when and why that character is countering.
Also, against mac either offstage or near the ledge, if you hit his counter with a cross-up attack, so that he turns to face the ledge/blastzone during the counter attack animation then the momentum will carry him off or away from the stage and can result in a gimp. Not sure if this works on a grounded mac, but it definitely work when he's in the air.
, and they keep countering, rush forward like you're going to attack, shield, and if they counter Rest them.
