This is a particularly nasty mission. You're tasked with cleaning up the water supply, which is being poisoned by Reapers. You have to close three valves to prevent the tar from entering the water supply. Each time you do so, some of the tar gets on you, temporarily reducing your Battery Cores and leaving you disoriented.
Clear the park[edit]
The mission begins at the south end of the park in the Neon. Your first objective is a fountain at the southeast corner of the park. Behind the fountain, you'll have a little talk and then be pointed to the first valve. Step up and press 
to close it.
You then have to try to follow Trish while disoriented, but there won't be any enemies to fight… yet. Ignore the giant Reapers that appear in front of you. After Trish helps you, head toward the other valves, which are located at the north end of the park.
Take care of any Reapers in the area and then make sure you're in full health by draining a light. Turn one of the valves. Reapers will spawn in front of you, so run away until the dizziness wears off and you can turn and fight. Try not to go charging in the direction of the other valve, or you will trigger even more Reapers.
Through the tunnel[edit]
Once you've cleared the area, approach the second valve. Once again, the choice is obvious. Good players will take the shot of tar to the face, while evil players will torture a civvie into doing it for them. There won't be any Reapers coming after you this time, but you won't lose the effects of the tar until the end of the mission. Recharge yourself and head toward the tunnel.
As you near the tunnel, you'll be dizzy again. Quickly step to the side so you won't be a sitting duck for the Reapers at the tunnel entrance. They won't come out after you, so wait for the dizziness to end and then step to the entrance and blast them.
The buses in the tunnel do not explode and work great for the trick where you create your own recharging station. Just pump them with Lighting Bolts until they crackle, then drain them. Use the buses as cover as you make your way down the tunnel, taking out all the Reapers.
When you have a dizzy spell and giant Reapers appear in front of you, start blowing stuff up. Target the cars lying around and take out the phantasms. Keep an eye out for electrical sources along the walls and keep yourself recharged.
At the end of the tunnel is a tanker truck guarded by a large number of Reapers. Shockwave the cars into them and blow the cars up, then overload the truck with grenades and Lightning Bolts. Your story mission now is to return to Zeke's Roof.