Chapter: 12, Checkpoint: 11
As soon as this checkpoint starts, you will have a large group of enemies above you, and one enemy on the ground in front of you. You don't need to use free aim for this, but if you need to use the locks, use soft lock.
As the roof starts to shake, dive forward towards the enemy on your level. Carefully line up your shots and try to kill him with a headshot. This may take a few tries, but you can just reply from the previous checkpoint if you miss.