Halo Wars 2

This achievement can be done in a Solo Firefight match or an online game with other players.

Whether you are attempting this achievement in your own solo match or with other players, simply refrain from deploying any additional units (Infantry, Air, Vehicle) onto the battlefield. If playing on Easy, this achievement shouldn't be too bad. As soon as the match starts, focus on building a healthy mix of Supple Pads and Generators on your main HQ and the two nearby mini-bases. This will allow you to have enough resources to begin building Turrets on your bases.

Because the waves in Firefight are randomized, you'll have to adapt to whichever "boss" you encounter on Wave 5 (sometimes it can be a single Scarab, other times a trio of Spartans). All in all, make use of the Infantry Units you are given when the match starts, and build as many Turrets as possible to help lay waste to the enemies for the first five waves.

Once Wave 6 starts, provided you didn't train/call in any additional units, this achievement will unlock.