Breeding diamond dragon isnt so difficult after all. Just got 44 hrs from daisy and charm without even trying. These pair is kinda undernourished as at lvl 19 with limited resources (only 4 small farms) 1 has to balance the budget between food, home and expansion.
At lvl 19 with only a handful of dragons and with so many dragons yet to be bred that any dragon is welcome. 4 col combo gives loads of choices of new dragons therefore more surprises and loads of fun.
But for a lvl 99 and above and having basically 80% of dragons in your island, getting a particular dragon that 1 desired seems like striking the lottery.
So for all those trying to get a diamond and dont want repeats.....my advise is....get rid of all the existing non-diamond dragons...then try a 4 col combo and any 'fails' wouldn't be repeats. Try it and enjoy the game! Good luck