Halo Wars 2

This achievement is best done on Mission 2, A New Enemy, but can only be attempted after collecting all fifteen skulls for the "Collecting a dream" achievement.

In the Campaign lobby, select Mission 2, "A New Enemy," turn on all fifteen skulls, and then make sure the difficulty is set to Legendary. When the mission starts, move all of your Units to the first enemy camp and take them out. You will then receive additional Units after clearing out the camp. Make strategic use of the Kodiaks you are provided soon after by moving your Infantry Units forward to clear the Fog of War. This will allow your Kodiaks to fire artillery at the AI's minibase. With the minibase destroyed, move forward to assault the larger base at the bottom of the hill. Keep in mind that you will receive reinforcements whenever you lose Units.

Once you're given access to build a base, do so immediately and begin building turrets as to protect it from potential enemy assaults. Construct one Generator at your base and then fill the remaining plots with Supply Pads. When you're set with the construction, move your Units to the eastern edge of the valley and attack the enemy base in the northeast. Once it is destroyed, you can build your own and construct additional Supply Pads here, too. Make sure to build turrets for defense!

With two bases under your control, allow yourself to sit still for a few minutes as to stockpile Supplies and Power. During this time, make sure to size up your army (Marines and Hellbringers alike). It's worth noting that you will have two Warthogs during this mission; whenever they are destroyed, they are automatically replaced.

After four or five minutes, head out to the final base on the ridge in the north, taking the eastern path up there (it's much safer to take this route as opposed to attacking from the southwest). It may be a wise idea to set your Global Rally Point Flag near this base during the assault so that any and all new Units you construct will automatically join the battle.

It may take some time, but after destroying the final Banished base, Decimus will appear. Focus all of your Units' firepower on the Brute Warlord and make sure to avoid the orbital strikes when he calls them in. Eventually, you will defeat the Brute and he will retreat, bringing the mission to a close.

The achievement will finally unlock upon successfully completing the mission, provided you were playing on Legendary and had all fifteen skulls activated.