Following Metal Gear's destruction, you'll receive a final radio call from Big Boss, revealing his true identity as the leader of Outer Heaven. He will challenge you to a final showdown. Your escape route will be blocked, forcing you to confront him directly.
Big Boss is a formidable opponent. He is armed with a powerful machine gun and moves quickly. The battle takes place in a confined area. The key to defeating him is to use your Rocket Launcher. Equip the Rocket Launcher and fire at him while dodging his attacks. He has a significant amount of health, so you'll need multiple hits. Conserve your rockets for this fight, as it's the most effective weapon against him.
After defeating Big Boss, the path to escape will open. Make your way to the designated extraction point. Congratulations, Solid Snake, you have successfully completed your mission and dismantled Outer Heaven!