Mission 1: Osiris
You can get this achievement on any difficulty in solo or co-op with a Fireteam of up to 3 players.
Once the mission has started, press
to command your team. You can command them to move to a position, attack a target, use a turret or a vehicle. You can also get them to pick up weapons and revive other members of the Fireteam (credit to NickMon68
If you are downed, pressing
to request a revive will also get you the achievement.
Commanding your team effectively, becomes more important on the later missions and on the harder difficulties. You can equip them all with power weapons (while providing you a limitless supply of Magnum ammo), have them cover you from Turrets, or have them fly around in Banshees while you select their targets from a safe distance. You can always get them to give the power weapons back should you need more ammo for yours. If you command them to attack a target they will spread out which will distract the target and allow you to hit the target where they are most vulnerable. If they do die, they will respawn after about 30 seconds. So if you are struggling at a certain point you can repeatedly send them to their doom while they slowly whittle down the enemy.