While heading to the staff lobby floor, make sure you pre-aim while still inside the elevator and before leaving since sometimes there are guards right outside and holding a shotgun which can instant kill you. But, that shouldn't be the case if you've already completed the Duggan Shipment prep mission as that would have weakened all the guards.
As soon as you are outside the elevator take an immediate right and then another right, heading towards two doors. Take cover and take some guards out before heading through those two doors.
Once you go through the doors take a left and another left where you will come across a long hallway that leads to the staff lobby. There will be a bunch of guards ambushing you along the way. Keep taking them out while carefully pushing forward until you reach the end.
Once you reach the end of the hallway take cover by the double door. This double door will take you directly out into the staff lobby, from where you can finally exit. Take cover here and take out guards in the lobby.
When you think it is safe for you to leave, head straight for the staff lobby exit which is through the metal detectors to your right as soon as you pass the double doors.
As soon as you exit through the staff lobby head straight to the racetrack and run on the right side of the track. Keep running until you reach the end, where the track will descend down towards your right.
Go down and then hop over the fence, out on the street and get a pedestrian car. After getting the pedestrian car head straight towards the sewer tunnels and stay in the middle of the tunnels until you've lost the cops.
These are the same tunnels where you entered the casino vault section from. After the cops are gone, continue through the tunnel and come out of the other end of the tunnel.
Once you're outside the tunnels make sure you do not go near the casino once again. Take a route to the buyer that is different to one which would cause you to go through the highway next to the casino.
So you need to make your own way-point towards the buyer and detour around the highway. Also, for those who don't know exactly where the sewer tunnels entrance is, refer to the picture below.
The red path is the highway which leads to high level buyers. DO NOT TAKE THAT PATH or else you will get 5 star wanted level again. Once you've successfully delivered to the buyer, your heist ends there.
By successfully navigating these escape routes and delivering the goods, you will have completed the Aggressive Diamond Casino Heist.
