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Worth: 15G

Description: In a single game, shoot down one of each alien craft.

How to Earn: There appear to be only six types of Alien craft that can actually be shot down by your Interceptors/Firestorms (number of aliens shown per ship reflect a "Normal" difficulty campaign):

1) Small -- "Small Scout," holding 6-7 aliens
2) Medium -- "Large Scout," holding 9-10 aliens
3) Medium -- "Overseer," holding 13 aliens (appears once, after Hyperwave Relay activates)
4) Large -- "Abductor," holding 11-14 aliens
5) Large -- "Supply Barge," holding 12-16 aliens
6) Very Large -- "Battleship," holding 22-23 aliens

As soon as you shoot down the sixth type of craft, the achievement will unlock.

Notes: I did not encounter a "Battleship" variant until after the Hyperwave Relay construction was initiated. Also, if you are initially unsuccessful or unable to shoot down the "Overseer" craft when first seen, don't worry, it will return as many times as necessary, until you finally shoot it down.