I'm not sure about Zelda, never played a really good Zelda player. I'd imagine a good one will space fairs and bairs really well, but Zelda's really really slow. Run away and shoot lasers if the stage is big enough. Most she can do is shield or use neutral B.
You have to beware of those fairs and bairs out of shield. Apart from the bair, fair, and nair, she's also got the fastest downsmash in the game I think, and her forward smash is deceptive. It's easy to let your shield down too early and get hit by it.
As for what to kill her with... I have no idea. I'd imagine the same things you use against Peach. She doesn't slide on the ground at all, so waveshine to upsmash seems like it would work the easiest.
I also have trouble with the Peach matchup. She has ridiculous aerial priority, so don't challenge her in the air, because the best you'll do is just trade hits.
Space drillshines well. If you miss her with the dair she's going to punish you hard. Do drillshines when she's at low percent because if you try a nair she can CC and dsmash.
Run away and laser a lot to rack up damage because she's really slow. Up smash and uair are the most dependable killers. Shine her near the edge of the stage to push her off, and then try to pull a shinespike or a bair. Her options are somewhat more limited off the stage, but she's still dangerous. One wrong move on the ledge and she can take your stock.
Raynex beat Vwins narrowly at a recent tourney, he probably knows more about the Peach matchup.