I’d like to add some personal experience. I tried the tips several times, finished the map and thought that I cleared everything, did it in less than 26 minutes and close to 28, saved minions and a structure for the last seconds before the cutscene, etc. And nothing helped until I saw this:
I tried again looking specifically for this void rift on the right edge of the beginning area – and it was there, not easy to notice, can be hit only by ranged units. Destroyed that, completed the map the same way, clearing everything on sight – and finally got rewarded with the achievement.