Nikolai in his mech suit will have a variety of attacks to damage you. He can shoot you with his guns at range, stun you with rockets, leap across the arena and stomp on you, and he can launch RAPS which will roll towards you and explode. Meanwhile, zombies and Manglers will continue to spawn and swarm you. Stick to the edges of the arena to avoid his ranged attacks, and be ready to strike when the time is right.
There are four weak spots on the mech: Two of them are glowing orange lights always present on the front of the mech, and two of them are glowing orange cylinders that pop up on his shoulders after some of his attacks.
Shoot these spots until they are no longer glowing to damage him. After all four have been destroyed, a more obvious weak spot will appear under the cockpit window for you to damage.
Defeating Nikolai starts the end cutscene and completes Love and War.