Casino — Loose Cheng
You need to rescue Tao Cheng from an abandoned motel in Sandy Shores and bring him to the Casino.
Walkthrough of the Sixth Casino Story Mission
Go to the ranch. There you need to get to the house where Avery is. It’s better to do it stealthily, so before starting the mission, you should get some suppressed weapons.
If you rise alarm, reinforcements will arrive, including several combat helicopters. In this case, the mission will get pretty hard.
There are several guards and cameras on the ranch. Their lines of sight are shown by cones on the radar. Stay clear of these and don’t make noise to avoid being spotted, or manage to neutralize the guards and cameras within a couple of seconds after you got spotted. The guards will also raise the alarm if they spot corpses. The optimal route to the target is shown in the screenshots below.
When you get to the marked house, Avery will try to escape by helicopter. You need to shoot down this helicopter while fighting off Buzzards and guards on Canis Kamachos.
Right after the cutscene, you will notice a Menacer, which is worth using. The driver should not get too close to the helicopters, as the turret rises to a limited angle. If you drive directly under the helicopters or in the immediate vicinity, the player controlling the turret will not be able to aim at them.
When you deal with the enemies, all that remains is to return to the Casino, and the mission will complete.
If you completed the mission for the first time as a host, you will receive GTA$500,000, as well as the armored Enus Paragon R — it will be waiting for you at the port.
Thanks to Mitch_Rajah for help.