Complete Episode 4 Achievement
Complete Episode 410
Complete all the chapters in Episode 4.
End boss: Vital Fortress
After activating the switch on the bottom level, have one person get into the VS sub. This person should just repeatedly shoot missiles at the boss. The other people should head to the top, grabbing any heavy weapons they can find. Aim towards the boss's red laser targeting system, this will give you the most bang for your buck.
The only attack you have to worry about is the torpedoes attack. It will shoot multiple torpedoes at you. This attack is easy to see coming, so just find some cover and you will be fine.
The video below was done on hard but the same strategy applies to Extreme.