When Nelf turns into the large eye, get close to the center of the eye when he is still in smoke form. Then use the electricity attack and he will die instantly. Note: You might still get hit while attacking. This trick works best with a controller with a turbo feature.
When you reach the Boss, duck and shoot your fireball. This works fast and is effective. This is easy since he uses his arms more than his legs -- he will only kicks with about one in every ten punches. Note: You will still get hit -- jump back and forth if needed.