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