Yeah, you can free run across the trees to the roof, but be sure to cut a sharp angle on the roof. If you take too long there one will die.
The trickiest part for me was once I got there getting Johnson to be my highlighted target. My first time I did it too quickly and launched after some random dude. --- PSN: YevinOrion | XBL: Dirty Machete
There's a path that leads above the target where you can kill him, be quick
I could try again from the checkpoint but there's always a scripted pair of guards passing by. So I just run past them and hope that the detection system doesn't go off?
Exactly what I did. I didn't even wait for them to walk off; I just ran right behind them as soon as space allowed and kept on going towards the ladder.
I did have to redo the mission though. My first attempt had me getting the target before anyone was shot, but it still counted as someone injured because the scripted event kicked in as I was midair. --- "Cat.The other white meat."