There will be a distress beacon from a building in the Amusement Mile. On top of a building Southwest of the GCPD you'll find a political prisoner. Talk to him and he'll be shot. Enter detective mode and scan the point where the bullet struck to trace it back to it's source. Follow the line back into the industrial district and examine the crime scene to find a bullet casing. After this, you will have to wait for Oracle to give you a call and tell you there has been another murder.
This is going to be in the Northeastern part of the Park Row area. East of the courthouse is a broken bridge and the new body is on that bridge. Check around the body again for another bullet hole. Scan that in evidence mode and then follow the trail eastward. On the building that the trail ends, you will find a tripod on the corner closest to the body. Search the tripod for evidence. Once you've done so, you'll have to wait for Oracle again.
The third victim is in the street NW of the words Park Row and SW of the courthouse. You'll actually hear some thugs talking about it and you'll have to take them out before you can check for evidence. Follow the line from the bullet hole over to the wall. There will actually be another hole where the line ends because the shot was ricocheted off of here. Examine that to get a trajectory for a building between the church and courthouse. On the corner closest to the victim, you can see some snow has been melted. Examine that.
On top of the NE substation, one of the three hatches will have a PDA behind it. Hack the PDA to find out Deadshot's location. Then you need to get to his position in three minutes and it's north of the Museum. Once you get there, you'll have to fight Deadshot. Stay crouched and move from cover to cover whenever he turns away. When you are close to him, wait for him to turn away and then rush him and attack him. Just keep striking him and he'll go down. Mission accomplished.
