Properly spaced f-tilt is really good for stuffing aerial approaches, depending on the move. However, sometimes it only beats startup. Like Falcon, for example, will trade with your f-tilt if you miss-space it with a knee or will just beat it. What you can do is see it's coming and f-tilt beforehand to beat the startup (hit them before the move comes out). This should make them respect your spacing and be more cautious when approaching. Unless they start to CC all your moves, but they should be doing that anyway.
As far as shield game, Sheiks is great. She has a big shield and her wavedash OOS is pretty good. Make sure you utilize that when punishing shielded aerials. Wavedash OOS f-tilt or d-smash is pretty good, if spaced correctly (basically all of Sheiks moves must be spaced properly or they are very punishable) or just wavedash OOS grab. Against spacies, try not to shield grab unless you download gaps in pressure, otherwise you're just going to get shined.
Also instant n-air OOS is godlike.
Hope this helped.