I just completed this achievement today, so I thought I would post some tips for some of the harder levels.
Going Deep - time.
I had a real struggle with the time on this mission. Firstly I would like to recommend following the videos (by GTASeriesVideos) posted in the video thread. For the shooting part of the mission in the car park, wait for the cue to explode your sticky bomb and then take out any remaining enemies. When the first two trucks arrive, throw a sticky bomb at the left truck and trigger it to explode. Repeat for the right truck. For the next few remaining waves, just throw sticky bombs in the middle of the ramp leading down into the car park and trigger them as the vehicles pass that point. Quickly shoot the guys outside to the left and then to the right and drive back to the objective (follow the path outlined in the video).
Oh, and don't do what I did. When you initially park in the space, don't hit the car infront and set off it's alarm, unless you want an instant fail.
Departure time - time.
When you arrive at the funfair, trigger the marker for the cutscene. Immediately turn around and head back to the road. Take a right and look to your right for a kind of gate (dark green?). Jump over here, only taking out enemies right in front of you. Look to your left for the drug container. Destroy this and jump back over the gate and make you way back to the top, where the van is parked. Take out the two guys and then peer round the corner to take out the other two drugs containers. This method saves you having to kill all the guys in that section. Then just proceed as normal with the rest of the mission.
Oh and a general tip to save time. When on the phone just cancel the conversation as soon as they start talking. Hope that helps.