Destroyers have two roles- destroying and contesting. With the ability to pump out S6 shots at 36” they are good for popping light transports or side armor tanks- use their jetbike like mobility to get side armor, while softening up objectives at range for advancing warriors.
End game they then shift over to objective contesting with their ability to turbo-boost and fly over models. Of course being such a good unit and one of the only real “threat” unit for Necrons they will get targeted by your opponent- and with only 15 max on the table they are not hard to take out even with WBB and T5.
So how to keep them alive? Cover of course, but more importantly using their speed to fully engage at 36”. Don’t zoom up and fire away at 24-12” and then get hit with rapid fire weapons. Stay back and pelt away. Also, as the game moves on, once you are down to 1-2 destroyers, pull back and hide. The remaining shots they pump out are not worth putting them at risk since you are going to need them to contest later.
Finally, consider solar pulse- you do have a lord with SP? THE best wargear in the codex behind veil of darkness and nightmare shroud…
Attach a lord to them and pop off pulse to protect you from a round of shooting- often good at turn four when you are putting yourself into a position to contest, or turn five to contest as a little extra insurance….