Here's what my take on the invisible hack is: In most maps with cliffs and water there are 2 heights that you can walk without the FLY hack. The ground level and the water level. If you edit your height close enough to the water level and deactivate the fly hack you should find your model on the water on both your and other ppl's screens. Then, the only thing left is to go to a place where you can see the mobs from below and attack with magic.
edit: oh and one more thing, I haven't tested this method myself because I guit RF for my exams time. Try it yourself and report on your finds in here