as long as he can comfortably use both Zelda and Shiek efectively, then Zelda has only two, count them TWO bad matchups that all Zeldas agree are bad for Zelda and Shiek:
Mr. Game and Watch
Marth
I personally find Peach and Snake bad too, but not everyone on the boards does.
The idiots of the Matchup thread would tell you to use ness or lucas... but Shiek puts a hurting on Lucas and Zelda doesn't fear ness.
Olimar can be tough for Zelda, but shiek can edgeguard kill him at shamefully low damages.
As for tips... hrmmm... Zelda's four directional aerials are all strong, but all have small, slightly laggy hitboxes which means that she's got some blind spots in the air so she's not at her best if you can get her there.... but be careful about chasing her up there, if she lands first, she could catch you in her godly Usmash.
Pick stages where Zelda will have trouble sweetspotting the ledge because she has considerable landing lag if she has to teleport avove the stage to get back on. And it makes her rather easy to punish.
If you can get inside her, try to stay there... Zelda doesn't repond well to being pressured... but don't get sloppy, if she connects with one of her aerials or utilt, you could be dead.
Zelda's a heavy hitter, but lightweight and not particularly fast. If you have a strong enough character, you can kill her early... but if you're light too, she can do the same.
Despite not having a sword, or other weapon, Zelda has MASSIVE disjointed hitboxes with awesome priority, multiple hits for punishing shields and a long duration making them hard to spotdodge. The also have little startup anc cooldown lag considering their power making them even more scary, but at least you can occasionally DI out of her multihit attacks.
if you have a shoddy recovery, I definitly would be careful, Dsamsh will be the end of you. And Dtilt can combo RIGHT into it, or another smsash.
Fear din's fire. it looks easy to spotdodge or shield, but a good Zelda has tricks with it.
I know he's a good charcter... but, don't play as DDD... Zelda destroys him... she also destroys fox and some other low tier charcters, but they aren't as important.
Shiek ***** ike and the space animals... so be wary of using them.
hrmmmm.... there's more but I just woke up and I have to get ready for my 12 hour day of work... *groan*