Enjoy your bus ride around town. While it may be tempting to ride the bus and chuck unlimited Grenades and Hammers, this is a sure way to get yourself and the bus blown up. Ride the bus until you see red on your mini-map, then jump off the bus and run down and take care of the gangs. By inching your way through the city like this, you will only have to deal with a handful of mobs at any one time and they will be shooting at you and not the bus. You can always run back to the bus and recharge from it without getting on top of it.
When you reach the bridge, you will have to ride the bus through the thick of the gang war. Just toss Grenades and Hammers willy-nilly all around and throw Shockwaves frequently. The bus will take a lot of damage, but this is unavoidable. When you finally reach the Warren, jump off the bus and deal with the leftover gangs from the bridge. Then, continue crawling your way bit-by-bit to the hospital.
After the cutscene, run away from the hospital around the back side of the building to the east (your right) and climb it. Throw grenades down into the street, recharge from the TV set, then ride the wire to the building to the northeast. Shoot the bombers on the rooftops to your north and throw more grenades into the street below. Especially target the machine gun turret.
Recharge and jump to the building to the north and keep throwing grenades in the street at every clump of red. When you have cleared the immediate area, recharge from the fuse box on the shanty and glide across to the hospital and climb to the top. As you reach the roof, use Precision to head shock the two Dust Men holding Trish hostage to end the mission.
You now have your choice of two story missions, which you can complete in any order. Some more side missions should also open up.