The Nox used three separate strategies in the destruction of Sectors thus far.
The Elven Domain was destroyed through pure strength. Nothing was able to resist the technological warfare the Nox were able to utilize at the time, especially not the universe's weakest race, the Elves.
Eden was destroyed similarly, but the universe still had the chance to fight back. The strategy the enemy chose was all-out war. The Nox took a huge loss in their technology and lost the ability to deploy destroyer starships after this war, but they were still able to come out victorious.
Nevertheless, unlike elves, who had colonies in several places, the Plant Races were essentially wiped out from the universe, at least in their complete, organized form.
Finally, the Giant Domain was taken with a strategy that still couldn't be explained. The best explanation was that they "rewrote the programming" of those targeted World Cores so they would naturally destroy themselves.