Here is the full list of all 15 Knight Squad achievements.
Accumulate a total of 1000 kills
Destroy a total of 5000 walls
Kill 3 knights in a single laser shot
Kill 7 persons in a row to obtain an Extermination
Ride a unicorn by stacking 2 horse power ups
Swing a single boomerang around for 5 seconds
Destroy 20 crystals in the Crystal Rush game mode
Stack the boots power up to max level (3)
Possess 8 minions at once
Defeat a single dark knight in the Dark Knights Army challenge
Complete the Dragon Alert challenge
Complete the Crystalor Challenge
Complete the Attack of the trolls challenge
Complete the Worms Attack challenge
Complete the Lightning Rampage challenge