Here is the full list of all 12 Gnumz: Masters of Defense achievements.
Destroy 10,000 enemies
Earn 240 stars
Destroy 5,000 enemies
Complete all levels of any episode with 3 stars
Complete a level with only one piece of ore remaining
Lose 3 times in a row
Complete a level without using spells
Upgrade any spell to the maximum level
Buy all the upgrades for one of the traps
Destroy 1,000 enemies
Use 200 spells
Only one enemy attacked the base