Here I will explain what are the most important steps in preparing the Diamond-Casino-Heist and the heist itself.
1.) Security Pass 2. You should definitly steal this one because Security Pass 1 is completely useless and you have to do very difficult hacks (what wastes a lot of time) if you own only Security Pass 1 or none.
2.) Duggans Shipments. Destroy these 10 shipments as many as you can, too. Otherwise the heist will become very hard after they hit the alarm and without 4 players and heavy weapons you have no chance to get inside the vault and outside again. So destroy minimum 3 shipments, so the guards lose their bulletproof helmets, are “only” heavy armored and have heavy weapons like assault Smg. Destroy 6, they wear light armor and medicore weapons like sweeper, shotguns and compact rifle. If you destroyed all 10, the guards have no armor and only have a pistol or a Micro smg.
3.) Patrol Routes. Do this mission too, otherwise you cant see the guards on the map and it would be not so cool if a guard kills you from behind because you didnt see him or if you srew them up in a stealth approach.
4.) Hacking. You cant avoid hacking in those 2 stealth approaches “Silent & Sneaky” and “The Big Con” and if you have done mistake 1.) you even have to hack in “Aggressive”, because you will waste your thermal explosives on these small secuity doors in the casino instead on the doors in the vault. To train your hacking-skills you should buy the hacking console for 425,000$. It seems expensive but if you do the heist more than 1 time you earn many millions of dollars and you can get the money back. Moreover, if u fail the hack, it will hit the alarm and the heist will become more difficult and you have less time in the vault to grab the loot.
5.) Cameras. Only important in stealth approaches like “Silent & Sneaky” and “The Big Con”. This prep mission is only avaiable if you own a casino-penthouse and did the 6 story missions as host. The result of the passed prep mission shows you the position of all security cameras and its field of vision in the casino. If u can’t do the mission, just be careful when you are walking around and take a look on the ceiling. If they see you or a dead guard, it will hit the alarm. You can destroy maximum 1 camera at the same time. Same with paralyzing them with the stun gun.
6.) Location of your Arcade. You should buy an Arcade in the city. No way Pixel Pete’s in Paleto Bay or Wonderama in Grapseed. These are worst because you have to drive a lot around while doing the prep missions and wasting a lot of time. I bought the one in Vinewood but I can recommend Videogeddon in La Mesa, too.
7.) Heist Crew. When you chose your crew for the heist, you should always chose the best avaiable hacker (Paige Harris or Avi Schwartzman). The better the hacker the more time you get to loot the vault. For Paige you need to own the Terrorbyte and for Avi you have to destroy all jammers in San Andreas.
As driver you can chose Karim Denz, he wants only 5% cut and chose the Sentinel Classic as getaway car. The gunner can be the cheapiest (5% cut) too, Karl Abolaji. Here it depends how many shipments you destroyed in “Duggans Shipments”. Even if you destroyed 9/10 shipments, the guards still have protective vests. So you should aim for the head (just like Thor on Thanos).
8.) Stairs instead of elevator. You should use the stairs instead of the elevator, especially if you do a stealth approach. But still be careful in the stairs! There is a secuity camera and 1-4 guards, depending on which approach you chose.
9.) Countdown in the vault. If you entered the vault without hitting the alarm…congratulations! Now you want to stay stealth. So don’t fail the hacks and leave the vault before the time is up. If you do the aggressive approach or already been exposed, you can ignore the countdown of cause. But dont stay too long in the gas!
10.) Escape from the Casino. Possibility 1: Escape through the sewer. Exit the casino by the exit you chosed on your final planning board because there somewhere your driver placed the getaway vehicles. Or leave by the personnel entrance, run over the racetrack, then down and jump over the fence. Now you reached a road where the Npcs are driving around. You can steal one of their cars and drive down in the sewer (you can find it in the south-east on the map). After you arrived in the sewer, there should be an entrance so drive inside and hide there from the cops.
Once the cops are gone, dont drive close to the casino or the police will hunting you again.
Possibility 2: Helicopter. In the west, on the opposite of the casino there is a police station and on its roof there will spawn a helicopter. Dont use your getaway vehicles to drive there and the cops still must hunt you. Otherwise the helicopter wont spawn. Drive around the police station where you can climb on the roof then climb over the ventilation shafts to get to the top. I guess the helicopter-possibility is the only option to pass the elite challenge.
Whatever you do, get off the cops and drive to the Buyer. As Buyer you should chose the furthest away one, because you will get more money. By passing the Elite Challenge, each player will get a bonus of 100,000$ extra.