Head to Andreas' yacht to begin this mission. The story goes that Andreas and the Assassinos have a problem. The DICS, or Drug Interdiction Counternarcotics Squad are making life hard for the gangs. Andreas wants you to take them out - it's a big responsibility as this involves taking out federal agents.
With the cut scene over, you have to head to the drug stash. This is located not far to the north. Head there - it's in a demolished building. Be sure to pick up the body armor and medkit that are also nearby before getting the sniper rifle which starts the combat section to this mission.
Once you get the sniper rifle, the DICS will begin attacking, and you need to use it to take them out. Note that you may have to drop your primary weapon by tapping the hand icon to pick up the sniper rifle. Stay in cover and only pop out once the enemy are in the open. Tapping the shoot button locks you onto the enemy while scoped, meaning you can just tap it again to take the kill shot.
Once you've taken out the first wave of enemies, the second wave will come in. Don't be afraid to move downstairs if they begin to flank you - providing you have decent guns you should be able to hold them off. Finally, another wave will arrive. Take care of them to complete the mission.
Reward: 8,000R$
You will then learn that Andreas has been killed in a drive-by and Marcelo is now in charge of the Assassinos. We will be wanting revenge, but for now, Chapter Two is complete.