Encounter: Market Escape (Chapter 11)
The perfect place to do this is the start of the Market Escape encounter of chapter 11. Make sure you put the difficulty on crushing! Enemies throw a lot more grenades on crushing. On lower difficulties it won't work so well. Crushing is unlocked from the beginning. You can change difficulty anytime in encounter select or in the pause menu.
At the start of the encounter there are 3 enemies. Kill the two on the left that come running. Once they are dead, the remaining enemy on the right will fire his gun at you and follow up with a grenade. Before he throws the grenade (while he is holding it in his hand) you must shoot him. Then he drops the grenade and blows himself up. Restart the checkpoint to farm more kills. If you quit the encounter your kill progress will be reset. So make sure you do 10 in one sitting.
To make things easier it is highly recommended that you enable the cheat "Bullet Speed Mode" from the bonus menu. It will greatly slow down time while aiming. This makes it much easier to time your shot. Usually, after the two other enemies are dead, he shoots at you once, goes back into cover and then throws the grenade.
Try to kill the other 2 enemies as quickly as possible with clean headshots. If you take too long, Sully might kill the enemy on the right.