Placing your mercs into Guard Mode orders them to aim and open fire on enemies who enter their field of vision. It is an auto detection and engagement mode that allows you to leave a merc able to respond without your intervention.
The game’s AI targeting system is pretty adept, but it is not as precise and effective as if you manually controlled each merc. Still, Guard Mode does offer an advantage in that it reduces the time to aim.
Because your mercs are actively looking for targets, it makes them quicker to shoot in any position they are in. There are several effective options to utilize this mode, and it is usually most effective when placing some of your squad in Guard Mode while manually commanding others.
For example, you can use it to have several soldiers guard an entrance or exit while other mercs advance on the target. Also, you can keep a group ready to ambush while another merc runs and lures the enemies into a trap Keep in mind that mercs with a higher perception attribute can see farther and engage the enemy at greater distances.
These are the best mercs to use in Guard Mode because they can engage the enemy at the greatest distances. In tough situation it is best to aim manually and control the flow of battle but if you have a hard time lining up a shot, you can use Guard Mode to allow the mercs to track enemies and fire as soon as they are “visible.”
Using Command Mode, you can pause the game and see the actual vision cone in green that represents each soldier’s viewable area. Make sure when using Guard Mode that you check this green field of vision to ensure they are facing the right angle. If you are using several mercs in this mode, you align their vision cones so that they slightly overlap but allow more coverage of the battlefield.