Story related, so you can't miss it. In Chapter 1 of Act 7, you'll be racing towards General Sarrano's ship to try and catch him before he escapes. A quick cutscene will trigger where a vehicle blows up and knocks you down. When you regain control of your character, you'll have to keep running towards the ship. Use the Energy Leash to pull down the ramp and boom! Easy 10G.