Belladonna!!
That is the obvious answer, Alex did not want to believe it but sadly that is reality.
He can handle Sandra and Belladonna perfectly alone, There will be no problem with him being able to manipulate them.
But if these two are together it's a nearly impossible job because these two are legit like fire and water, Both are standing opposite fractions with one another.
While Sandra is FL with her own female lead halo, Belladonna is villainess with her own villainess halo.
Both of them are natural enemies of one another and because of that both need completely different ways to deal with them.
He for Sandra goes with the stimulation route making her slowly get used to him being with other women, As FL has an immense inhuman level of possessiveness for her man, She likes to be one and only women in her man's life.