Gameboy Wars 3
Total ATK: Used ATK * ((100 + Level Value + Flank Value) * HP / 100)
Total DEF: Used DEF * ((200 + Land Value + Level Value + Flank Value) * HP / 200)
New HP is determined as such: min(old HP,ceiling((Total ATK - Total DEF)/(Used DEF * 2)))
Advance Wars : Days of Ruin
Simplified:
BaseDamage * (100 + AttackBoost) / (100 + DefenseBoost)
Precise:
floor(floor(floor(BaseDamage * (AttackBoost / 100 + 1)) * (AttackerHP / 10)) * (100 / (100 + DefenseBoost)))
Zone boosts both Attack and Defense boost by 10 where applicable.