Is that the mission where you have to kill a chap for Martin and then take a photo of the corpse?|
If so, it's a pain in the arse. I managed to do it with Larrybong, just two the two of us. You can shoot down into the compound from the roof of the Skyscraper with the 400,000 flat. Police choppers will spawn, but they don't really hurt you.
The men will look like blombs, but you can get chest and headshots from that distance. Whittle them down. When you get next to the flat, more people will spawn but again you can take them out, escape the cops, go back and kill more etc.
I think it might be bugged, such is the difficulty.
www.hatchetjob.com - more than games. "Seemingly trivial, surprisingly deep"