Made to love you: tell her that she was born at the exact moment that he starts talking to her. Her worldview will be softened somewhat, and a singleminded devotion to Gil will rule her decisions.
Assistant: try to maintain her original personality to the best of his abilities, including her cold devotion to the mission. Ask about her mission, and tell her her own story back to her save that her mission was to assist and serve him in his role as a beta tester, and she fell for him in the process.
Concubine: let the great be laid low. She’s a pretty elf, and we don’t want her getting jealous, so just let her know she’s a devoted harem girl that learned magic in order to be more useful to him.
Sulky Mistress: like the last one but with a little more bite. Put less effort into suppressing her more antagonistic traits, so she is more likely to work to maintain his attention.
Damsel in Distress: tell her she was a princess that he has saved from her kidnappers (her subordinates) and that she’s falling madly in love with him on the spot. Could be fun.
Dicknotized: tell her her own story as well as he can, but that when she got a taste of his dick she completely abandoned her old mission to be with him, happy to go wherever as long as she gets his dick.
No. Not wasting it on a small time mage. Save the Waters for a bigger name antagonist or higher value waifu and just stamp one of the lesser mooks after making a deal for access to mana. They’ll have much less to teach Gil about magic, but less opportunity cost.
Stamp one of her minions in addition to brainwashing Candress. Why not bring along both, and in doing so show the underling both the carrot and the stick.
Fuck Elves in the least literal sense available. Just kill Candress if she’s going to be difficult, and all her mooks with her.
I’m alway open to suggestions. If someone makes a suggestion I like enough/feels most like Gil, I’ll probably just go with that unless there’s an overwhelming consensus on the poll.