I don't know much about the specific frog, but if you get a tank of that size (which I think you should if you can), you do probably want something more than a handmister. You'll be misting your hands off if you are to keep the humidity up. A waterfall does not a very much to the overall humidity.
A fogger (Zoomed's ReptiFogger for example) or a misting(rain) system (Exo-Terra's Monsoon or the Mistking for example) would be a great idea, if not a must.