The way I went about it was to lead the mastermind towards the barracks by keeping him on the bottom right of the map at all times. Don't let him get within the firing distance of the anti-personnel turrets or you'll be done for. Once you have the turrets in sight, take over one of the GDI tanks and destroy them that way (if the tank doesn't survive, keep on taking over subsequent tanks until you have the turrets out of the way), but keep the mastermind safely away from the action.
Once you wipe out the first two or three (can't remember how many there are) turrets, move your mastermind towards the barracks, then teleport the two alien tanks and take out the turrets guarding the barracks. After that "take over" the barracks with the mastermind and train an engineer, lead him over to the power source and take it over. The engineer will not be shot at and fireworks will ensue
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