You have to make a long run through the dark to get to the sewer where you'll bring the eighth and final transformer back online. Stick to the rooftops; you'll encounter very few First Sons up there and you'll be able to pick off the ones on the ground without exposing yourself overly much to fire. If you pick your way carefully, you can stay at a high level and avoid much of the danger below.
The last transformer[edit]
This time, when you drop into the sewer, you are immediately confronted by a couple of gang members. Crouch behind the generator in front of you and use Precision to take them out. Jump from pole to pole to get to the transformer and press 
to reestablish the circuit.
You'll receive the Lightning Storm Power. This awesome incarnation of Cole's powers is unleashed by holding 
. The Storm starts near you and runs farther away as you hold down the button. You can steer the Storm by tilting the controller left and right. Lightning Storm eats up a lot of Cole's energy, so save it for extreme situations or when you have a good power source nearby. Lightning Storm only works above ground, so you don't get to play with your new toy right now.
Work your way over to a tunnel and watch for the First Sons that pop up at the other end. Carefully shoot your way forward until you arrive at a generator you can crouch behind. Use Grenades or Hammers to take out the First Sons to your right and far ahead of you, then quickly jump and glide over to the generator to your right. You can now see more First Sons that were to your left before. Take them out, then jump to their platform.
Now it's just a hop, skip and jump to the sewer exit. Watch out for the falling platform! As you leave the sewer, you get to unleash Lightning Storm on a bunch of turret trucks. Take out the trucks and then use more conventional weaponry against the foot troops.
Get the substations running[edit]
Now you have to re-energize three substations above-ground, and it's dark, so not much chance to recharge your batteries. You can use the occasional car, but not much else until you get to the substations. Save your energy for when you reach a substation; don't waste it on the journey. Make your way toward the first objective marker; use the rooftops to avoid the turret guns (and take them out from on high) and watch out for the mines on the roofs. It may actually be easier to die here as you'll start back at the exit from the sewers, and most of the random enemies that spawned into this area earlier will be gone.
When you reach the substation, you'll find a small block building near a gas station. Clear the enemies from around the area, drop down and charge it up. Now you get a charged panel and you have to keep First Sons away from the substation while it recharges. Since you have unlimited energy, just spam Lightning Storm. Make sure you heal once in a while.
Repeat for the second and third substations. If you are in need of additional XP, this is a mission where you can rack up serious amounts of XP by playing a substation defense over and over again.