Here is the full list of all 15 Knight Squad achievements.
Possess 8 minions at once
Accumulate a total of 1000 kills
Destroy 20 crystals in the Crystal Rush game mode
Destroy a total of 5000 walls
Swing a single boomerang around for 5 seconds
Ride a unicorn by stacking 2 horse power ups
Kill 7 persons in a row to obtain an Extermination
Stack the boots power up to max level (3)
Kill 3 knights in a single laser shot
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