If you mean punishing his OWN normals, there are several ways to do that:
The first thing to keep in mind is that none of his normals are three frames. This means your jab can punish his own jabs. You must be extremely careful with this, however, as his MEDIUM attacks are all 0 on block and therefore completely safe, unlike some of R.Mika’s mediums which can put her at a bit of a disadvantage if the Nash gets jab happy.
The second thing to keep in mind is that a LOT of Nash’s love to zone and love to jump attack or go for cross ups. This means that you’ll want to generally make him uncomfortable on the ground by throwing out throws and poke pressure via hk and shooting peaches (,they can go over his sonic booms bypassing his zoning attempts,) so you can make his air game more predictable and your anti-air that much stronger (wingless airplane, standing medium kick, and lady mika are all good options here.) Alternatively, you can try making him uncomfortable in the air using her jumping medium kick, jump back light punch, and in some cases if he tries to counter jump from too far away heavy kick. And if he doesn’t react to your jumping, almost all your jump ins are going to be safe or push you far away enough that he can’t do anything that isn’t super slow. Be careful with this though, as he has decent anti air/air to air options and this is normally better for conditioning/baiting things out.
The more common Nash’s tend to do a string of attacks that can be ex peach or jab punished depending on what it is (which you can cancel into light peach for a knock down.) More technical Nash’s I have found, tend to try and go for cross ups. Crouching medium punch is a good way to punish that, and is a fairly necessary skill for all match ups honestly. Timing wise you really want to keep Nash at a /little/ distance as If you mean his specials, you either have to block or get something so on reaction that you’d need superb.
To answer the more specific part of your question, his heavy attacks tend to get used in his strings. If you are patient enough to look for them during them, you can jab punish those. Additionally his crouching medium kick is 0 on hit and therefore you shouldn’t be too discouraged to retaliate with a jab or even a throw if you are close enough if you get hit by it.
In reality though, a big advantage of Nash is his movement and his specials. He has great set ups for these due to his speed and dash speed. He specials have great pressure when used correctly as most of them cannot be retaliated against once you start blocking them. These are more so the sorts of things you’ll have to be good against to get a more honest game with Nash. His normals are definitely something to watch out for, but at the same time if your defense is good enough they are not necessarily going to be your biggest threat, especially when he has meter or v trigger ready.
Anyway hope that answered your question at least a bit. Happy monster nashing.
