Halo Wars: Definitive Edition

This is really doable on all difficulties. If you are just going for this achievement play on easy and put on any skulls that will help you out.

Short version:
Get an fully upgraded hornet army. Go to your Spartans at the star. Kill the colony to the right. All colonies will show on your mini-map. Kill all colonies quickly with your hornet army. If they are in hibernation they will turn pink. To get the achievement kill the main flood structure in the upper right corner while the lesser colonies are in hibernation.

Detailed version:
You are safe at your base. Only some weak enemies will attack, but nothing your turrets and elephants can not take care of. So take your time to build a fully upgraded hornet army.

Build the following at your base: 6 x Heavy Supply Pad, 2 x Reactor, Armory.

Upgrade both reactors. Research +10 units at your armory. Destroy your armory and build an Air Pad. Fully upgrade your hornets. Queue hornets until you reach max population, you should have 16 hornets.

Now go with your entire air-force to your Spartans at the star. Put 3 Spartans in 3 hornets. To the right of this locations is a colony. It looks like this:



Destroy it. The Spartans will locate all colonies at your mini-map. There will be 6 dots in total, the main colony to kill is on the upper right corner. Save this one for last! To get the achievement all colonies have to be in hibernation when you kill the big one. If a colony is in hibernation the red dot will turn pink.

Now quickly go from one colony to the other with your entire air-force. First kill the "goo" shooting flood structures that are close to the colonies to keep your hornets save. Then 'kill' the colony. Rinse and repeat as fast as possible at the other colonies.

If a hornet gets destroyed, replace it at your Air Pad, however, don't let your main fleet wait for them to be finished but move on to the next colony asap and send the newly build hornets after your main fleet. This can easily be done by selecting all units with RB and using your right trigger to scroll to select all hornets at once.

When all 5 lesser colonies are in hibernation (pink at your mini map), go to the main colony at the upper right corner and kill it quickly. It will already be low on health since shooting the other 5 colonies in hibernation weakens the main colony. Before attacking save your game! If a lesser colony "awakes" Serena will warn you and the pink dot will turn red. Swiftly instruct your hornets to send it to hibernation once more before killing the big colony.

If for any reason you kill the big bad guy without having all colonies in hibernation reload your save and try again.

The best way to do this is make as large of an upgraded hornet army as possible, then try to clear out all enemies at each of the five flood bases, avoiding killing the main base until all other things are dead will make this go better. In any case, you have to get all five of the main colonies dormant and before one of them comes back to life, so you need to kill them all as quick as possible.