After you get the windmill online and generate the power surge the mechs will come crashing through the gate. As you battle them, be sure to save as many plasma and stun shots as you can. A dog will attack in the courtyard.
If you bought the plasma shot damage 2 upgrade you can kill the dog with only 2 stun shots and 5 plasma shots on normal. Otherwise follow the same pattern as last time. Keep the dog stunned as much as possible so it's head mounted gun won't shoot you.