How difficult is it to learn to fly a revolution quad line kite?

I learned dual line first than happen to know quad line. Got a lot of interest in quad line its super fun. Coming to your question , you can learn the basics by watching the online tutorials . They teach every thing from installation to basics to get started

In a word,,,very difficult; but quite rewarding if you stick at it. There is a slightly more controllable option called the Benson Airbow made by legendary kite make Tim Benson. Revolutions are best flown with a team as some spectacular tricks are possible by virtue of the fact that you can stop the kite in mid air and fly it backwards.   They are quite twitchy and you need a decent wind to learn how to fly them.