There could be different kinds of attack that corresponds to different armors> i.e. "Anti-Air" Attack, "Torpedo" Attack, "Artillery" Attack, etc... corresponding to "Anti-Air" Armor, etc...
So planes have low "Anti-Air" Armor, but high "Normal Attack" Armor. Then, Infantry (which has low "Anti-Air" Attack) can hardly damage a plane, while an anti-aircraft gun (which will have high "Anti-Air" Attack) is much more effective.
All the units will attack by default the corresponding attack type> i.e. Infantry attacks planes with the weak "Anti-Air" Attack, but will attack other Infantry with their slightly stronger "Normal" Attack.
How does this sound?