I'm well-studied in this matchup because I play it often and live in Lain Land. Honestly, you're crazy if you think this matchup is an easy win. Here's my collaboration of information about Ice Climbers.
Rule #1: Ground yourself, children.
I'm not joking. A double jump can mean the difference between three stocks and two. Ice Climbers have a better, more damaging U-Air juggle than us.
Rule #2: Avoid using Jab/Plasma Whip and limit Flip Jump/Kick.
This may seem difficult, but IC's get a free stock after a PS'd Plasma Whip. Furthermore, our third jab can be shieldgrabbed.
The last thing you want to be is predictable and Flip Jump does just that. It creates a predictable trejectory, much like a roll. Side note: Blizzard stops the flip function of Flip Jump; express caution, especially at the ledge...
Rule #3: Get them offstage ASAP.
Squall Recovery and Attack getup are punishable with D-Smash and they seem to be the most frequently used options by IC mains. Their roll get up and normal getup are terrible without the use of desyncs. You can literally get a free D-Smash there too. It all comes down to prediction, but alllowing them to even grab the edge is where you messed up. Edgeguard them to hell if you plan on winning. No, I'm not joking. This matchup is tougher than you make it out to be, guys.
Rule #4: IC's Are Unplankable.
Flip Jump stall until the Blizzard stops. Be careful though, Ice Bergs can gimp your Flip Jump if you mess up your timing.
Rule #5: You have no safe approaches.
That's right, don't even try it. Your best approaching option is B-Air because of IC's terrible grab range, but even then... Well, let's just say that you have to be close enough to the IC's for B-Air to be viable.
If you are positive that you can catch the IC's while standing still, you may attempt a Plasma Whip approach. Keep in mind that you're taking a huge risk, though. The reward is a shot at killing Nana or a free Dash Attack or Jab when the climbers drop their shield. Even Blizzard outranges Plasma Whip though, so good luck catching them off guard unless you wave bounce it. Actually, that's a good idea that requires testing. Can we wavebounce a Plasma Whip through a fullhop blizzard?
I would expect IC's to be shielding a lot against us because not only will they not expect a ZSS to grab, but they can interrupt a grab with Nana/Popo's jab. Your best option is to fire an uncharged paralyzer in this position. It will stun Nana if they powershield and encourage them to drop their shield if they don't. Punish the dropped shield. :]
Rule #7: Juggling
Juggling completes itself with edgeguarding in how to beat IC's.
Don't allow them to land and don't allow yourself to get above them. When above IC's, you're running a very good chance of getting juggled or grabbed . You can head for the ledge, but our ledge game is useless there.
Plasma Wire is useless as well, simply because you don't want to risk spiking them to the ground for the reasons stated above. Take note that their D-Air has incredible range and sends them straight down, so it is tough to consistently U-Air before they D-Air. Often times one U-Air is enough to separate the climbers, but Popo isn't going to give you his woman without a fight. Practice, practice, practice.
Rule #8: Tilty Whaaaa-?
U-Tilt is risky upon being shielded, moreso in this matchup than others, obviously.
F-Tilt has a larger range than their grab range when spaced horizontally. The only thing they can do OOS before you can react again is possibly F-Tilt. They can PS though, so be careful.
D-Tilt is amazing here, especially when you're behind the climbers. It is your best bet at setting up a juggle and effectively killing Nana. Note that I'm not sure if double D-Tilt shield pressure can be pivot grabbed by the climbers, but I'll try to find out asap. I know you guys like to wait for the opponent to drop their shield.
Rule #9: Single Popo is no push over.
If you're not Zero Suit Samus. Popo has a D-Throw chaingrab at 0% that does work against ZSS. With that said, Flip Jump is always an option of the opponent doesn't buffer their grab. What makes single Popo so bad against ZSS is B-Air. Regardless of percentage, B-Air approaches to B-Air edge guarding will always beat him out. He can't recover low, so you can predict his single Ice Berg jump upon recovery and boost jump into Plasma Whip. If it misses, just punish the Squall Hammer he's about to use to recover with D-Smash. If you predict that he won't Squall, flip jump stall at the ledge and allow him to fall to his death. Simple.
Rule #10: Counterpicking and Camping.
Ban FD and platform camp when all else fails.
Strike Yoshi's Island (brawl). I believe it is our second worst neutral in this matchup. The tilting platform creates space for them to pressure you (with U-Airs) into dropping off or through to the stage. Even something as miniscule as that can open you up for a grab. Most Ice Climbers ban Brinstar, but both Brinstar and Rainbow Cruise should be easy wins for ZSS. If you win your neutral, you should win your set.
You can camp IC's, but only if there are platforms. Otherwise I wouldn't expect them to approach you. Ice Berg spam can cause you to become frustrated on flat stages (Seriously, ban FD). If you jump, you run the risk of getting caught in the air or being grabbed. If you shieldwalk forward, you run the risk of being grabbed. If you approach with Plasma Whip, you run the risk of being grabbed.
It's okay, I hate this matchup too.
Rule #11: **** Nana, don't get grabbed.
Nana plays melee in Brawl. Have fun.
40:60 Lain/Ambrose/Meep (good) Climbers
50:50 Ice Climbers
90:10 Popo