Technique is the part of scoring determined by how well the player protects their units. For AW1/2/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. 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.
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.