If you have someone to help you, a diamond wheel on a Dremel (or similar tool) works well. You just have to go in short spurts and dunk the coral in water every 10 seconds or so to cool it off. The reason you might need an extra set of hands is, in my experience anyway, it's hard to only have one hand to hold the coral... If it's something branching, you'll be fine but otherwise might need a way to hold it securely while you use the saw.
Another option is a Gryphon Aquasaw.