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