Phase 1: Mutated Nemesis
This is the final confrontation with Nemesis, who has mutated into a grotesque, multi-limbed creature. He will attack with powerful slams and acid projectiles. Your primary objective is to activate the rail cannon. To do this, you need to push the three power generators into position. While doing so, Nemesis will constantly attack. Use the pillars for cover and dodge his attacks. Shoot the exposed eyes on his body to stun him, giving you a window to push the generators. The Magnum and Grenade Launcher (Acid Rounds are effective) are your best bets.
Phase 2: Rail Cannon Activation
Once all three generators are in place, activate the rail cannon. Nemesis will be stunned by the initial blast. Now, you need to fire the rail cannon at him. This requires a short charging time. During this time, Nemesis will try to destroy the cannon. Continue to shoot his eyes to keep him stunned and prevent him from damaging the cannon. Fire the rail cannon multiple times until he is finally defeated. This is a battle of attrition and precise timing.