Atlantica Guide To Guarding
As many might have noticed, you are able to shift turns to other people in your party if their AP (Action Points, the Green Number by their feet) is over 100. Due to how the multi-mercenary system works in this game, you are allowed to have a total of 9 mercenaries at any given point (Atlantica online Gold). The following turn, 3. The next turn, and for every turn thereafter, you can control a random 5 people.
As you may have noticed, some mercenaries are Greyed out, meaning they were not randomly selected the turn to do anything. Just because they are Greyed out does not mean you can not use them - this is a common misconception. If their AP is on/over 100, you can effectively Guard out of a Green mercenaries turn to turn another Mercenary from Grey to Green, enabling you to use them. From what I've noticed and from personal experience Guarding not only benefits because you switch to another mercenary you want to use, but the other mercenary is in a Guarded stance, allowing extra defense and making them take less damage from direct hits.
This can be applied to any mercenary over 100 AP, like I stated. So lets assume Merc I, who we'll say is a healer, needs to heal Merc A because he is damaged, but he didn't get randomly selected this time (even though his AP is over 100) - this does't mean Merc A has to die.
Also, please keep in mind you cannot re-guard a turn back to a guarded character - Unless after guarding they still have 100 or more AP. Guarding itself takes 50 AP!
In my opinion, guarding right, and using the [D] key to move your guys around in battle is as important as your formation and how you attack the other person. This tends to be truer in PVP, because you are fighting another human element who also knows these tricks of the trade - and they will use them, a lot.