This is a cumulative over the campaign and while fight against other players and will not be a problem if you go for all other achievements.
At first, you have to do some missions and level up a little until be capable to use air units.
On the second campaign: Deceive Mardonius, you can use air units on missions 3 and 4. They are easy to win, but try to use some better units and always have Boons (artefacts for power/defense and attack you can create) to use.
Mission 4: they have only 25 points of attack and 35 for defense.
Mission 5: they have 41 points of attack and 43 for defense.
Both mission have 5 enemies so if you want to boost this here repeat 100 times. To every fight you have to pay 6 with faith.
When you have destroied the 500th you will got the achievement.
