Be quick and drive to the GPS point. You've got a limited amount of time to complete this mission, so book it. When you reach the target, you'll find the alley guarded by myriad security guards. Not that it really matters. Just gun 'em down en route to the locked door in the middle of the alley. You'll need to hack your way in, but that's not hard.
Once you get inside, then things turn bad. A flash grenade rolls in and a pair of baddies, one with a shotgun, roll in. Gun them down quick and they should drop their guns and a bit of health. Leave the health if you don't need it, 'cause many more enemies will roll into the room. Be quick to move around and gun down the enemies, picking up the health and armor drops as you need them.
When the throngs of guards stop pouring in, Mel leaves her small enclosure, letting you inside to grab a bat. Equip the bat (important: the bat does not equip itself) and use it to hit Mel once to knock her down. Move to the marked switch at the bottom-right corner of the screen and a cut scene takes over.
You're now behind the wheel of a van and need to chase down another van that the cameraman has run away in. Catching up with him can be tough, but he's not the best driver so you'll eventually get your chance. Ram into the van and try to wedge it against a wall, then uload your gun fire to quickly destroy it. Once the van catches fire, leave it be and it'll blow up in due time. Drive Mel to the GPS target to end the mission.