Here is the full list of all 14 Class Kingdom achievements.
Gain more than 100 exp in a level
Do 200+ damage to a single monster
Destroy the first dragon
Do 100+ damage to a single monster
Choose a class for the first time
Choose Fighter as your first class
Destroy the first boss in the game!
Finish the first map
Finish the 2nd world successfully
Finish the 4th world successfully
Find the code hidden inside the first world
Choose Archer as your class
Choose Sorcerer as your first class
Enter the SECRET code in the first world