This was the hardest part of the entire game for me on Hard. Here's how I killed Derek in the warehouse for the achievement.
Take the car that is at the entrance of the warehouse, and drive slowly inside until you see one of the 2 guys on the ground floor straight ahead. The guy in front of you will move toward the left, but will then try to dodge you car by heading right. This is you chance to run him over or pin him to the crates on the right. Now I'd get out of the car with the other guy on the other side of the crates, take cover and take him out.
Once the 2 guys on the ground are taken care of, take cover behind one one of the columns underneath Derek, facing the stairs to the office. There are probably 5 or 6 guys left, but you can easily pick them off when they come down stairs and rush towards you.
Take your time, and if no one else is coming down, sprint toward the stairs to see if anyone fires at you. Take the shooter out, and then run after Derek, who will back himself into a corner so you can execute that SOB.