Play Chapter 7 under the Survival difficulty setting. While progressing, conserve as much ammunition and trap parts as possible. Take advantage of the existing spike traps to kill various enemies. If done correctly, you will encounter the Keeper boss with about 30 traps and a decent amount of ammunition for your various weapons. Start the Boss battle as usual by shutting down the poison gas. After closing the final valve, remain in the area and keep fighting. The Keeper boss will keep respawning from the safes about thirty times. Keep defeating him to get 2,000 Green Gel each time. With fully upgraded damage on your harpoon bolts and pistol, only one hit with each of those weapons will be enough to take him down. By the end of all the fights, you should have collected about 70,000 Green Gel. Note: You cannot damage Keeper for a few seconds after he spawns. To get more trap parts, use the Keeper's barbed wire mines. Simply wait for the Keeper to deploy his mines then kill him. Then, use the free time during his respawn to disarm the mines. Note: Keep a sniper rifle or shotgun in reserve in the event that you run out of harpoon bolts.