Halo Wars

As the achievement description says, this is for completing the first mission on any difficulty. As you'd expect, it's a very straightforward mission with tutorial elements - it practically tells you how to complete the mission.

Your task is to direct Sergeant Forge's warthog, rescuing marines on the way. You'll occasionally come against shields which are only passable by marines on foot. Therefore it is advisable that you keep a couple back out of the fight for this task - once they're past the shield, you can attack the generator to allow the rest of your units through.

A video of the mission is below (credit to GamersThrone).