First part; Stay underneath one of the tunnels and poke your head out occasionally to shoot him in the eye with the Laserkraftwerk, then you shoot his arms where the rockets come out of. Once you have destroyed his arms where the rockets come out of, you'll have to then run underneath the London Monitor and shoot his engine while avoiding machine gun fire. Occasionally you'll have to shoot him in the eye but that won't be a problem, since he has no rockets to shoot at you.
This method applies to every difficulty!