It' also hard to keep your fingers pressing the FN whenever is ready. Last night while doing ubers I noticed a curious function of the CM with mouse, keys and skills, in my personal case I used to keep the click press down so the Twisters keep coming out. Never though that this will work for spells with cooldown like FN, but it does!
You still need to be smashing the keys, but at least you don't need to worry about having the boss always frozen.
By the way, with this method the FN will keep casting every time its not in cooldown. The other spells aren't important as FN, so you can relax and smash the buttons not as fast as used to.
Probably a lot knew this all ready, but at least me and some of my friends didn't knew.