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