Airdodge the Din's Fire.
And no, you won't be going through the upsmash from above with any attack. If you want a short hop approach that may work you'll have to use a short hopped double bair. Zelda also punishes rolls fairly well. She's tough for luigi, you'll have to get creative. You'll have to read your opponent a bit no matter what you do. I like shield dash grabs, when she makes a mistake I can punish with a grab that is. Also, when in fireball range, you can spam it until she is forced to jump and try to approach you(which is weak, Zelda has powerful aerials, but it doesn't make for good approach game) or use Naryu's Love to reflect. Try to predict the Naryu's and go punish the lag on it with another shield-dash grab or running upsmash. If you don't spam the fireball, fireball to tornado is nice, as is landing tornadoes across the ground into her or other times where using the tornado is a surprise. If she's trying to be less predictable, she may quick dash attack. If you can, shield, and then you are nearly guaranteed an up-b firejumppunch out of shield. If goes for a shield dash grab, expecting you to shield like it was a dash attack, spotdodge, and you can almost always follow up with an fsmash on the lag she has if she misses a grab. Almost all of her attacks are similar in startup time and ending lag. All of them are pretty fast, and most of them have nice extended hitboxes. You can start to learn the timing on rolling and spotdodging to get past the range and be able to punish on most of them. Shielding is a problem vs. the fsmash, because it's multi-hits will easily knock luigi, who will slide, away. You really have to time a spotdodge or roll instead, with the roll being the better of the two as the spotdodge will usually leave Luigi still too far away for a hit. Otherwise, if hit by the fmash, learn to DI out of it, because you can escape with only as little as 4% damage taken.
Her uair is a very reliable killing move for her. Keep this in mind when returning to the stage. Given her upsmash and uptilt, it's quite risky to come from the air towards her. If you do, for some reason, you obviously need to airdodge, but also airdodge in a way that the airdodge will finish by landing on the ground, and during that airdodge buffer it into a shield. Or more simply, press shield to airdodge and keep holding shield to bring up your shield first frame you can when you hit the ground, and don't let go until Zelda's attack is completely over. This is important or otherwise her multi hit move like her upsmash will still hit you.
Zelda is a lightweight. Fsmash KOs start around 80%, up-B KOs start at around 45%, upsmash KOs start at around 90%. Any higher than that, and fresh backthrow, bair, sweet spooted sex kick start KOing too. She also has very weak downwards aerial options and little in the means of aerial moves to defend herself save a lucky sweetspot. Getting her up in the air can usually mean you can safely chase for an aerial string, which should let you build up damage fairly easy as long as you get her up at least once. Other than that, KO her quickly with Luigi's KO power and her lightweight nature.
