Here is the full list of all 22 AX:EL - there are Air XenoDawn achievements.
Complete mission 1
Shoot down 10 enemies
Kill your first enemy
Visit the hub 100 times
Visit the hub 50 times
Visit the hub.
Visit the hub 250 times
Kill 50 opponents
Shoot down 100 enemies
Kill 250 enemies
Destroy 500 enemy vehicles
Send 1000 enemies to the "otherside"
Complete Mission 2
Complete Mission 3
Complete Mission 4
Complete Mission 5
Complete Mission 6
Complete Mission 7
Complete Mission 8
Complete Mission 9
Complete Mission 10
Complete Mission 10 in a special way