vs ZSS
She has a lot of stuff that can lead to ridiculous damage combos, and potential 0-deaths, particularly if she gets you into a juggle, so try to DI out as best you can, but it's not easy if your opponent reads you well. That said, she has a lot of weaknesses.
Get in close, but just out of her jab range. She can't really sideB or grab you at that range, and she has nothing fast enough to do much about it there. Her fair and bair are good spacing tools and combo weapons, but not very good panic buttons. In fact, she has no panic button. Get her on the defensive, and she's boned. She has no sex kick, no shield grab, no upB out of shield. And obviously, she's very limited by her tether only recovery, so take the opportunity to gimp her when you can. Look out for a dair or upB spike though. It also helps to have a small character, as her hitboxes tend to be have very specific heights, so a small target can throw off her approaches.
vs Ivy
Offensively, he has some very safe moves. Look out for ivycopter and ftilt, bair, fair approaches. He also has the copter to recovery with, so try to kill with low angle shots so you can gimp his recovery. Do what you can to try to bait out his unsafe moves, uair, dair, leaf storm and usmashes, or wait till he gets into a razor leaf throwing pattern and surprise him with a jump in.
vs DK
no clue. I don't play DK, nor do I play against anyone who does