Quick Purge is basically an instant-kill sneak attack.
Crouch and move up behind an enemy undetected and press L2 to kill them.
The screams of the enemy won’t alert nearby Visitors, so as long as you stay out of sight, it is easy enough to take down a whole group of enemies with stealth kills.
Using quick purge on enemies knocked to the ground during combat also counts towards the total, but normal melee core grabs on still standing enemies do not.