This can be done anywhere against policemen and any enemies. After the first few story missions, you will learn how to perform brutal takedowns. At the end of a melee combo, hold B and release it at the right time. You will see a short quick time event with a button prompt. A small circle will fill up and you must release the button at the right time to perform a special type of takedown. There will also be an audio cue when you have done it correctly. Simply cause some chaos, let the police spot you, run up to them, perform three melee attacks in a row, then hold B. You must perform three of these finishers in a row, no other kills are allowed. It is recommended to bring some adrenaline shots to refill health and play on the Easy difficulty.