In Crackdown 3 there are a total of 9 targets representing three major factions that support Terra Nova's evil activities: Logistics, Security, and Industry. To begin with you kill the Lieutenants targets of each faction which is a diverse group ranging from prize-winning chemists to decorated former army officers and then eventually you move up to the department heads. Once you have eliminated all three of those you will able to take on the final boss: Elizabeth Nemand, the evil dictator of Terra Nova.
To discover the locations of each of the targets you will need to clear out specific objectives in the open-world that have minimum requirements. Although just doing the minimum requirements for them will shorten the game you will find that unless you are proficient in fast-paced third person action games you will not have enough firepower and movement skills to defeat the final boss. It is therefore advised that you complete the side objectives for each target as these will provide additional facilities to destroy enabling you to increase your firepower and movement skills.