Denyce is the first survivor you encounter and you have to keep her with you for the campaign until Case 7-2 when you fight Sgt Boykin to unlock this achievement.
Try to be a high level when you do this and always carry lots of health. You should get the leadership magazine from Rag Mags on the second floor of the Royal Flush Plaza. Give Denyce the LMG - it is found in the Yucatan Casino in the elevated grassy area across from the tiger enclosure and next to the restaurant.
Leave her outside of the area when you fight psychopaths during the story (i.e. the Twins in Case 5-2). When you have to go to the roof of the Fortune City Hotel to fight TK, leave her in the Royal Flush Plaza bathroom. To get the mission done quickly mix 2 Quicksteps beforehand (wine + wine or coffee + coffee creamer) and try to beat TK quickly. Use the Protoman Arm Cannon against the helicopter if you have it unlocked as it is very effective. Once TK is defeated you will be teleported to the safehouse, head out to the Royal Flush and heal Denyce.
It will makes things easier if you don't fight many psychopaths during this playthrough. Try not to hit Denyce much either as she may defect. Save often too so if she does die or defect, you can reload the save. Good luck!