Belle
Rumple would know more than I, dear. I'll answer you regardless but I would still ask him.
Maleficent was powerful back in our land. More powerful than I ever was. The only way that I was able to subdue her and retrieve the dark curse Rumple had created was to find her weakness. It just so happened that her one weakness was the black unicorn that she had taken on as a pet. We duelled for some time but I restrained her when her gaze was split between myself and her unicorn. There you have it. Pure honesty. The Maleficent who is here now only vaguely reminds me of the one from our land. If she has a weakness I'm not sure how we'd go about discovering it, and I do mean we. I will do what I can to fight against her but using magic is out of the question. The only advice I can give now would be to not incite her anger. Despite the chaos that was her arrival she's been pretty benign in nature, but that can change.