Technique is the part of scoring determined by how well the player protects their units. For Advance Wars 1/2/DS, simplifying the formula in Campaign mode, you must not lose more than about 20% (not exactly 20%).of your units that had been deployed in battle to get a perfect 100. So for 5 units that is deployed for the player in Campaign, you can only lose 1 and so on. In War Room, however, it is reduced to about 10% (again, not exactly 10%), meaning that for 10 units produced, you must not have lost more than 1.
In The Final Battle! in Advance Wars, only Andy's score is counted.
In Advance Wars 2 missions with multiple armies, only the main army is counted (for Blue Moon, Yellow Comet and Green Earth) or the army that destroys the final object (Hot Pursuit and Final Front).
In Advance Wars: Dual Strike, all armies' technique scores are counted. The technique scores of all armies are added (there is no limit of 100 when calculating), then divided by the total number of armies present to get the average technique score.
Advance Wars 1/2/DS[]
Technique = M + 100 * (T - L) / T
M = Map-based Lost limit for 100 Technique T = Total Units L = Lost Units
Advance Wars: Days of Ruin[]
Technique = (E + J) * 125 / (T + L)
T = Total Units L = Lost Units E = Enemy Total Units J = Enemy Joins
The above is averaged with 100 if it passes 100, and maxes at 150.