The 'Submarine Recon' setup mission guide for The Bogdan Problem (Act 2 Doomsday Heist) in GTA Online is the final setup before the Act 2 Finale, utilizing the Strombergs acquired in 'Prep 4a' and Torpedo ECUs from 'Prep 4b'.
This is one of the easiest missions. You and your friend need to get into the Strombergs and head to North Chumash. You need to destroy a convoy here using your Strombergs. You have the option of using either the machine gun or homing missiles. You can switch to your homing missiles and destroy all the vehicles in the convoy which includes both land vehicles as well as the helicopter. The homing missiles will automatically lock on so you don't have to worry about anything — just shoot.
After destroying the convoy, you and your friend need to head over to the Paleto Cove. You will have to submerge inside the water and will get the option to convert your car into submersible mode. Head to the sea mines around the submarine and destroy them all. Your friend will also be doing the same thing. You don't have to worry about anything since the homing missiles will automatically lock on to the targets, just be careful not to get too close to the sea mines.
