Fast travel to Warehouse, head south into the tunnel to find the radio station. Radio for a ranger to be deployed to you and then had back into the warehouse area to make enemies spawn Get the enemy's health low and then press "square" on them but do not seize them completely, they will stand still so your ranger has a good target. Just press "seize" again and again without capturing them and ranger will do the rest.
One ranger doesn't need to get all 5 kills, just has to be 5 kills total from any ranger.
Tips from Edzio117 and KleptoJack.