oze6000, I'm going to agree with sasook and say that low % kills aren't really all that common. Once you get opponents to 130-ish, though, they should be dying pretty fast.
If you're having trouble edge guarding/edge hogging, the only way you can really get better is through practice.
A few things to keep in mind when attempting to give an opponent off the edge a hard time:
Killing an opponent is RARELY worth you losing your own stock (the exception is of course when you have a stock advantage, you are at very high, and they are low on %, all 3 should be true).
Going for straight forward edge guarding techniques is a good start, until your opponent reacts to it properly. Straight forward edge guarding techniques would be something like jumping out and doing a b-air or f-air off the stage.
Keep as many recovery options open to you as possible. It's very easy for your own inexperience to gimp yourself when going for "tricky" edge guards. Basically, don't throw out your down+B liberally going for the spike. Same goes with ledge hopping when an opponent is capable of grabbing the ledge and reversing the situation on you.
For the most part, after you establish that you WILL go off the stage to make it hard for people to recover, it becomes a mindgame. Try and make them predict your actions wrong by using visual cues that make them expect you to do something differently (line up horizontally with them for a b-air, but delay it until after they air dodge, or air dodge the attack you think they will try to use).
When you approach them while they're offstage, do they air dodge as soon as you get in range to attack? Do they attack you? Do they have a counter that they try to use? Look at the what their options are and consider whether it is truly worth attempting to edge guard. Ganon is a particularly dangerous character to edge guard or get high vertical kills on while he's in the magnifying glass, as he has his down+B which has enough KB to hit you off the top, and his side+B (for when you're not sure how far away he is from you at the side of the stage) is a great self-destruct kill for when he's at high % and you are not (for both resetting a match and maintaining a stock lead, if he has one). Most characters aren't quite as threatening with their anti-edge guard arsenal, but I was just giving some examples where I had to modify my own edge game to continue to gimp them (air dodge his side+B, and just not go for vertical kills up high against Ganon).
Some tools you have in your arsenal are the up+B boost jump (or whatever you guys call it, where you jump higher while using up+B during the jump ascension), b-air, f-air, side+B, the b-air stage spike, the d-smash to b-air stage spike and/or flipspike situations. ZSS has a lot of tools to use, more so than most other characters.
The edge hogging is really just all from practice. You can drop from the ledge and down+B back into a ledge grab, or you can drop off and wait and then side/up+B to grab the ledge from afar to deny someone else from grabbing on. This takes a lot of practice and knowing the recovery moves/timing of different characters. Particularly tricky opponents can really screw you up if you get greedy in edge hogging/guarding, and inexperience will also be a large factor.
I don't feel like I've put enough emphasis on it, but critically thinking about the risk and reward of edge guarding is important. Sometimes the risk is just too great. If you are too greedy, you will get punished. It's happened to everyone. When you get greedy and are punished for it, reflect on what you did, and think of whether it was a bad decision on your part or inexperience, and think about how you can learn from it.