Halo Wars 2

This achievement can be done in a Solo Firefight match or an online game with other players. The solution below will assume you are playing alone.

In Terminus Firefight, the sole objective is to protect the Terminus from being destroyed by the enemy. When the Terminus is taking damage, you will receive an audible and on-screen "warning" that the Terminus is under attack. For the purposes of this achievement, since you need to protect the objective from Waves 1-15, should that warning ever be displayed, quit out of the match since you will have failed the requirements for this achievement.

In the pre-game lobby, set the difficulty to Easy. As for your Leader, any will do, but it's recommended to choose a UNSC Leader such as Jerome or Sergeant Forge. Now begin the match. Once you have player control, begin building three Supply Pads at your main HQ. Make note of each wave's enemy patrol (the dotted line on the mini-map) and use your already-available Infantry Units to repel their attack. If necessary, go ahead and deploy 2-3+ additional Marines. As you begin stockpiling resources, build another Supply Pad on the nearby mini-base and then upgrade as many Supply Pads until you run out of Energy.

When your Energy is depleted, you should have enough Supplies to build two Generators on your main HQ. Construct these buildings and continue to hold off the enemy's attacks. While you are defending the Terminus, begin building 2-3 fences to the east and west of your main HQ (focus on the direction the enemies are spawning most frequently). Should supplies withstand, upgrade the fences once, then again into Fortified Bunkers. You can then begin to fill the Bunkers with Marines. In addition to this, be sure to build Turrets near the Bunkers and at the base of the Terminus (upgrade all Turrets are the Anti-Air variant).

With this initial setup, you should have little issue holding off for the first 7-8 waves. Because you will continue to stockpile resources while defending the objective, make sure to build the second mini-base with an upgraded Supply Pad, more Anti-Air Turrets wherever you can, and even more Fortified Bunkers and Barricades. Moreover, once you have a stable economy, you will want to upgrade your main HQ and build an Air Pad; Hornets become a near-necessity for those pesky enemies that either attempt to destroy the bunkers/barricades or the Aerial Units that tend to make a beeline towards the Terminus.

All in all, be sure to pay attention to the path each wave's enemies will take. With plenty of Anti-Air Turrets, Marines camping out in Fortified Bunkers, and a sizable army of Hornets, you should have an easy time reaching and defeating wave 15 without the Terminus ever having taken damage. Remember, though, if you hear or see the warning that the Terminus is under attack or taking damage, quit out of the game and try again because continuing on for another fifteen waves will be fruitless.