How to kill Derek and Steve with out getting killed?

You leg it staight to the back of the warehouse and take care of the two guys there. Then if your still alive , wait for a couple of guys to run down the stairs spray them, then throw some grenades up onto the 1st floor without getting firebombed untill everyone is dead

I was having a lot of trouble with this and I didn't stock up on ammo or grenades before this mission so I had to try another way. I drove straight to the back of the warehouse , took cover behind the car I was in and shot all Derek's goons. I then ran up the stairs to his office, you can take cover up there and once I was there I didn't get hit once. I shot Derek through one of the windows, walked to his desk and got the cash, it was as simple as that hope this helps!!

Actually you can drive to a vehicle into the back of of the warehouse. It worked perfectly for me and I didn't get hit even once.