Here is the full list of all 20 NC Tower Defense 2 achievements.
Kill 20 Enemy Infantry Units
Kill 200 Enemy Infantry Units
Kill 2000 Enemy Infantry Units
Destroy 10 Enemy Vehicles
Destroy 100 Enemy Vehicles
Destroy 500 Enemy Vehicles
Destroy 10 Enemy Air Units
Destroy 50 Enemy Air Units
Destroy 200 Enemy Air Units
Destroy 25 Giant Enemy Vehicles
Build 100 Towers
Allow 150 Towers to be Disabled
Repair 50 Towers
Sell 50 Towers
Begin the first level
Finish 18 Levels
Encounter The Secret Weapon
Finish final level on Normal or Above Difficulty
Spend a Minute in the Training Sim
Finish all Levels with Highest Medal