Some spoilers ahead! You have been warned.
This boss fight takes place after you complete the "Nest Lab" stage.
Before we start: for loadout I'd recommend Flame Rounds (this boss is extra vulnerable to them), the Magnum, the Pistol and the Shotgun (for taking down the zombies that spawn).
You will come to a large arena, and after a quick cutscene will have to fight nemesis again. It's probably the hardest boss fight yet. You need to have mastered dodging here as his attacks will seriously hurt. On Standard you can usually take just one or two hits before you die. Nemesis also has a "giant arm attack" with serious reach. Dodging these is pretty much essential so try to practice.
First off, you just need to damage him enough for the cutscene to start. Carlos appears and will help you for the rest of the fight by spotting where nemesis has gone.
Nemesis will start running in circles around the edge of the arena. You need to wait until he stops behind a generator, then shoot the red squares to shock him (I recommend using the pistol to conserve ammo on your other weapons). When this happens he'll fall to the ground and you can shoot at his exposed red "heart" to damage him. You don't get long to do this so be quick and unload into it as fast as possible. I suggest Magnum or Shotgun.
You need to repeat this process a number of times, shocking him and then shooting him when he falls to the ground. Each time he jumps up to run around the outside, two zombies will appear in the arena and need to be dealt with. Make sure you don't get grabbed from behind once you're focusing on Nemesis, this happened to me a couple of times. Sometimes Carlos will help out by hitting Nemesis with the crane.
Later on in the fight Nemesis will occasionally jump from the edge of the arena onto you. Watch out for this.
Ammo and health can be found in boxes around the edges of the arena if you need it.
Eventually you'll do enough damage to him to finish the fight. He'll fall off the side back into the arena and a cutscene will play.
The following video gives a good overview of the fight but sadly misses of the final part. Could be helpful if you're really stuck though. (Credit to "SweetJonnyCage" on youtube)