On the mission "Pursue Swifty" you will come into a a big clearing where your introduced to the "Shield" mechanic. Your darkling will pick up a car door and distract the enemies. Simply run up ahead of him with the fully upgraded gun channeling activated and spam bullets. You will get 5 or so kills in a few seconds then restart your checkpoint and do it again. I did this all in one sitting without ever really using GC previously. Remember to restart the checkpoint before it saves.