Power is the amount of enemy units you destroyed in a single turn. For Advance Wars 1/2/DS, if there are 38 units your enemy deployed, you must destroy at least 4 units in a single turn to earn a perfect score or if 74 units, 8 units in one turn. This is usually the easiest category to score in, unless your opponent can mass produce units quickly.
Power = 1000 * D / T
D = Destroyed enemy units T = Total enemy units
Technique = [(D + 100 * K) / A] - 100
D = Damage dealt in total K = Kills dealt in total A = Attacks executed altogether
Counterattacks are not factored in at all.
The above is averaged with 100 if it passes 100, and maxes at 150.