This guide will make this achievement so much easier for you!
First off a few tips:
Once you get in the chopper the timer going down does not matter anymore, it will go under 3 minutes maybe due to the chopper flying away and loading times but this does not affect your actual playtime, as long as you make it into the chopper with 3 minutes or more each time you will be good to get this achievement.
ALWAYS aim between the center of the torso and head area for maximum damage when killing the criminals and the cops.
Go into the options and turn on steady camera, this will make your life easier.
Now for the guide:
Now as soon as you spawn run to the right along the side of the train that's empty and enter the train at the first door you can, when you get to the door on your left there will be a cop, I would hip fire him to save time and once he's dead you want to go into the door on your left of where you entered and start grabbing the cash at the back most part of this room.
Once the other criminals enter this room you want to open fire immediately and take out the 2 that will be in the room with you, then run to the front of the train straight ahead of you immediately after they are dead and kill the 3 guys in the front of the train, ignoring the money (for now). Once they are dead run back to where you were but stop at the middle carriage and there may be the remaining 2 guys in this room or they'll be outside the train to the right of you (on the side you initially spawned into this match on) quickly kill them (they may have money but don't grab it until you have the rest, continue reading) and go to the front of the train and start grabbing all the money and work your way back to the room where you killed the first 2 guys, grabbing all the money along the way (now grab the other possible money outside). You should have about 4:30-4:00 minutes left at this point (maybe more), depending on how good/fast you are, as well as 4 million dollars or more depending on if you killed everyone which you should have no matter what, if you didn't just restart, the spawns can sometimes differ but it's rare and this guide totally revolves around killing everyone which isn't hard.
Once you've got all the money proceed to the exit on the side you spawned initially and run up the big broken ramp part of the bridge, there may be 1-2 cops here, possibly one by the broken piece of wall outside the train, one up by the van/car on the right side of the ramp your running at the bottom and possibly at the top of the bridge on your side. Just keep running and and kill any cops that you have to along the way, from long range you can take quite a bit of damage, you may get knocked down but you aren't dead. (Bare in mind that the criminals and cops/S.W.A.T are extremely deadly up close and will kill you in a split second if you are not careful)
Now ESCAPE and repeat this guide once more and you are done.