To build Terry and Clay's toughness, they have to complete missions and gang wars with you. Once you become leader, you can call Terry or Clay on your phone and ask for backup. If you complete the mission that you are on with their help, their toughness goes up 8%. If they start the mission with you, their toughness will go up 12%. When you complete a gang war, their toughness will go up 4%.
To call Terry or Clay for backup, press view your phone, then press on Phonebook. Scroll down and press again on either Clay or Terry and then press again on Backup. You don't have to call both of them to get backup, once you call one, both of them will come.
You can easily get this just by playing through the story. Call one of them on every mission to boost their toughness and to get help on the mission.
If you don't get this by the end of the story, complete all the gang wars to get their toughness up the rest of the way.