Thing is, your opponent will know - from the instant that you turn to Zelda, that you just want the kill. Don't fish for the kill, particularly her upsmash. The move, while it's a great shield poke, will result in you being punished if they shield all the hits and you're in enough cooldown for them to follow up with something.
Upsmash is great for punishing a well read air dodge, because of the multi-hits. Up tilt is alright for punishing landings because she arcs it all around her. Also if your opponent is at above 95 damage, there's a good chance that down tilt will pop them up (unless down tilt causes them to trip, in which case you can down smash right after). Popping them up from down tilt is a guaranteed upsmash/up tilt for you to follow up with. If they are at REALLY high percents, down tilt will pop them up too high - you could try an up air, but it's more avoidable for them.
If you think they'll get up from the ledge you can try to space a fsmash. There are also ways in which to get a guaranteed lightning kick, but that's more complex, particularly with learning how to nair cancel with Zelda, which I can show you sometime if you would like