This can be done in any game mode. To unlock this, you must melee (
) five enemies to the ground then press
again on each of them to "finish them off", all within 30 seconds. This is rather easy when you’re overwhelmed by weaker enemies.
There is a foolproof way to get this at the beginning of Mission 7, 'Forsaken City'. There are five enemies already on the ground and you just have to press