Provided you told Johnny to beat it and saving Sandra during the first visit to NYC, Sandra will be at her father's hotel during JC's second visit to the city. Go to the 'Ton Hotel and then from the lobby go into that admin room to the left and through the door you'll see in front of you as you enter it.
Talk to Gilbert and when he asks for a weapon, refuse to give him one. Once the conversation with him is done, Jojo Nice will spawn where the NSF had taken hostages the first time you were here and walk to where the Rentons are. Have a weapon you'd find effective in this close-quarters fight ready. The conversation will take a turn for the worse and Jojo will open fire at Gilbert. Let Jojo kill Gilbert and then kill Jojo before he does the same to Sandra. If you are actually careful and quick he will aggro towards JC, reducing the chances of Sandra dying.
Once he's dead, the achievement will unlock after the dialogue with Sandra ends.
You can actually unlock two more achievements during the above sequence, provided you have at least one bottle of Zyme on you, read the solutions to both of them to understand what you'll have to do: