Halo Wars 2

First thing's first: invite a friend or another player into your Skirmish lobby. Once you have at least one other player, set both of the AI's difficulty level to Easy and make the following game setting changes:

• Gametype: Domination
• Fog of War: OFF
• Starting Supply: 9999
• Starting Power: 9999

Now start the match and once you and your partner have player control, begin upgrading your bases and building your armies. The main goal here is to capture at least two of the Control Nodes in order to start scoring points. Given that the AI is on the lowest difficulty, you shouldn't have any issue at all in holding both points and repelling the enemy's attack. The match may take a while, though, so if you want to speed things up, go ahead and capture the third Control Node and press onwards to attack the AI bases. Whether you're playing as the UNSC or Banished, it's recommended to storm the AI bases with plenty of Air Units and heavy-hitting, ground-based Vehicle Units.

Once the match ends, you and your partner will both unlock the achievement for defeating the AI.