Here is the full list of all 20 Banshee Force achievements.
Finish level 1
Finish levels 1-5
Finish levels 1-10
Finish Levels 1-15
Finish levels 1-20
Build your first tower
Finish a level with less than 10% health on your last city
Have 100 or more missiles in your stockpile
Build 5 or more towers of the same kind in one level
Finish a level with no shield on the last remaining city
Finish a level without killing a single enemy ship
Finish a level protecting 2 cities from being destroyed
Finish a level without building a tower
Destroying an enemy ship by hand with missiles
Finish a level with no damage done to towers and cities
Finish a level out of missiles and not able to buy more
Hit two missiles with one missile of your own
Hit three missiles with one missile of your own
Hit four missiles with one missile of your own
Hit five missiles with one missile of your own