Like in the previous battle against the Nemesis you’ll want to do a quick loop around the area to pickup the following items, Shotgun and Handgun Ammo, Green Herbs, and some Explosive Rounds. Once you’ve collected everything you’ll want to equip the Grenade Launcher and begin firing Flame Rounds at his head until he transitions into the 2nd Phase, now he’ll run along the walls for a short perioud of time. Wait for him to stop running and then quickly fire an explosive round at him to temporarily stun him.
This is the perfect time to equip your shotgun and begin focus firing on his heart. Repeat this cycle until the Nemesis is defeated once again.