P.S. I should prelude this by saying that I haven't actually played a Zelda. Mostly theory, so take it with a grain of salt.
It's a tough battle in all aspects. Nayru's Love is safe on use most of the time, especially when your fireball is helping it. A lot of Zelda's attacks out range Luigi, which is especially harsh because that's a big con to him. I see this as a fight where it's necessary for Luigi to bait, as Zelda does have some extra lag on a bit of her attacks. Fade outs, dropping downBs, and SHFF Nair are all useful when approaching. Play defensively, with plenty of mind games. Empty hops, running past her (is there a term for that?), and just general mindgames are needed. It's definitely a fight tilted towards Zelda in theory, but she is punishable on a lot of attacks, so try to take advantage of that.