I thought the other solutions here were good, but the best way I found netted 11 Knight kills every ~90 seconds or so, letting me grind from 50% to unlock in a maybe a couple hours.
Easy difficulty, Infinity, checkpoint Charlie. Rush through for a bit until you enter the Mantis (past the section with the tank/hunters). You'll know you're at the spot when you, a) get in the Mantis, and b) Cortana says, "The path to the maintenance causeway is jammed".
You should get a checkpoint as you're jumping into that Mantis. Then just proceed through the door and down the hallways, killing a total of 11 Knights, and reload the checkpoint. The first Knights spawn in a group of 2, then 2, 1, 2, 2, and 2. After those last 2 reload the last checkpoint to avoid getting a new one - my checkpoint was literally as I was getting in the Mantis so it made grinding very easy.