Oryx and other ascendant hive put their souls in their own personal dimensions so that they can't die. Vex just friggin time hop across everywhere I guess (maybe they meet the precursors before they became precursors??) oryx will just take the flood and command it to his will. And warlocks are damn near invincible due to how much power they can fling around. I mean a warlock can create a goddamn supernova from their hands. That means that 1. They have enough power to destroy solar systems 2. They are able to contain and withstand said power. Halo can definitely kill guardians given enough time. But halo can't access hive throne worlds and they can't stop the vex because they are also coming from a different dimension. And if something were able to kill an ascendant hive then oryx could either take whatever killed that hive, or the hive can just do evil shit that reflects the ascendant hives personality and bring it back the hive dude back to life.