She is fast AND she is strong, but she is also extremely light. Just so you know, ZSS stun gun does not have very long range and if you don't approach this can make her extremely impatient. I am guessing the ZSS you're coming across who are the type who run up, down grab you and follow up with an up b. As soon as you block the projecile or spot dodge it, tumble backwards and her grab should miss you. The recover is so bad on this, you can likely get a Koopa Klaw in for good damage.
Best advice too is to know two things, what attacks not to use and staying grounded. Never use a down-b unless you know the ZSS is dumb or for some reason has decided to park herself underneath you. Even then her up-smash out prioritizes out butt slam, so it's asking for trouble. Second, do not use your forward smash as if the ZSS is any good, she will not be hit. The exception to this of course if you somehow break her shield. Third, DO NOT engage ZSS in the air. Stay on the ground. Out of shield grab her if you're fortunate for her to park in front if you and either pummel her, followed by a forward tilt, or launch her into the air. Your Up-smash goes through her down air and can potentially K.O her at around 80% if you charge it enough. Saying that, try to punish ZSS as she comes down via Up-smash or a shield and klaw/grab. The only exception to this is a fair from below, but only if you can safely assume she isn't going to down-b your face.
For general engagement, the tumble loving ZSS, neutral a (The double jab) can work wonders to catch them off guard as they'll likely be passing you by. It also provides a small knockback and potentially open a dash A (drop kick) into the air to rack up some damage. But always remember when she is coming close, prep the shield and get ready to spot dodge (the grapple).
Another exceptional ally we Bowser's have which has thankfully transferred perfectly from Brawl is your down tilt (where Bowser strikes out with his fists in two swipes while crouched). It has good reach and it hits like a truck. In fact it can K.O ZSS at roughly 95%. I use this as a substitute for actual smash attacks to get her out of the ring. In addition to this, fire. I've probably said this to the point of being blue in the face, but approaching by fire while she is trying to stun you with her gun can work wonders. Not only does it cancel out her gun's projectile, it can also rack up some good damage.
Finally, of course if you have been grappled and she chucks you into the air, hold down the control stick to descend quicker and MASH the L (Or R, depending on your configurations) to air dodge any potential up-b or air combo. Knowing how to and when to air dodge is also crucial as I feel Bowser has a minor edge over Samus on the ground. She is fast and hits surprisingly hard, but damn we can hit harder and can definitely take more.
Good luck.
