Here is the full list of all 37 Alien Robot Monsters achievements.
Fire 500 missiles from orbit
Deploy 128 reinforcements
Instakill 50 robots
Destroy 128 robots with fire
Gain all 111 stars
Complete all Planet X levels
Defeat final boss
Gain 90 stars
Gain 75 stars
Defeat Mountain biome boss
Defeat Swamp biome boss
Build 800 towers
Destroy 99 robots with satellites
Gain 50 stars
Gain 40 stars
Build 400 towers
Deploy 32 reinforcements
Defeat Wasteland biome boss
Gain 30 stars
Destroy 128 robots with Atomizer
Build 200 towers
Field 256 marines
Save the natives
Gain 20 stars
Destroy a robot with native arrows
Defeat Forest biome boss
Destroy 100 robots with snipers
Use 1337 hand grenades or mortar rounds
Fire 256 sniper rounds
Build 50 towers
Destroy 100 robots in melee
Stun 384 robots
Gain 10 stars
Field 64 marines
Destroy 50 robots in melee
Field some space marines
Start your colonization of Planet X