Back to the topic of the giant robot powered by diesel-electric:
Another improbable way to power this beast it is microwave power transmission. We were thinking either several mobile armored power stations or even better (worse) putting the generators and transmitters on board very large aircraft (say a fleet of 8-10 Antonov An-225s) and have them continually descending on the machine in a large airborne chain (meeting up later with a chain of refueling aircraft double to triple the number of your fleet because these burn a lot of fuel!).
The main problem here is you've already built a death ray of enormous capability. Why bother with the robot when you can flash fry the contents of a tank (or the lungs of an infantry division) from a very safe distance? The robot would be useful in drawing fire away from your death rays I guess.