Here is the full list of all 42 The Onion Knights - there are Definitive Edition achievements.
Kill 1000 Infantry
Kill 25000 Infantry
Kill 50000 Infantry
Kill 800 Armored Soldiers
Kill 20000 ArmoredSoldiers
Kill 40000 Armored Soldiers
Kill 600 Throwers
Kill 15000 Throwers
Kill 30000 Throwers
Kill 400 Chargers
Kill 10000 Chargers
Kill 20000 Chargers
Kill 1000 Paratroopers
Kill 25000 Paratroopers
Kill 50000 Paratroopers
Destroy 100 Catapults
Destroy 2500 Catapults
Destroy 5000 Catapults
Kill 5 Dragons
Kill 50 Dragons
Kill 100 Dragons
Level 10 Up
Level 30 Up
Level 50 Up
Training 1 AfterTraining
Training 10 AfterTraining
Training 20 AfterTraining
More 5 Fire
More 25 Fire
More 50 Fire
RichMagic 5 Power
RichMagic 25 Power
RichMagic 50 Power
Specialist 1
Specialist 10
Specialist 15
PerfectDefense 60
PerfectDefense 150
PerfectDefense 300
Extreme 200000
Extreme 2000000
Extreme 20000000