first, let me ask....how do you get 20 gold in one day, lucky you!
personally i try to battle every single hour the first and second day to get past the first 15 rounds quite fast as i konw i will get stuck one day at least in a few dragons.... i usually speed rounds in those dragons, normally 15,20,25 i speed them up by half hour, so i only spend 2 gold, i cant tell for sure of course but usually after i speed up, i tend to have better luck in beating the dragon, but that could be just random.
also, i do what a lot of people have been suggesting. for the harder dragons i dont use 10 of my best dragons, specially those that have 10% chance for pretty much any dragon you use, i use babies and non-important dragons in between, keeping in mind which dragons im supposed to use next round.
and, last, i keep it in my mind that im ready to ultimate the last or before last round, i wouldnt do it way before beacuse i cant afford to ultimate more than one round and there's no sense for me to ultimate round 20 and then never make it past 25...but that's just my personal scenario.
its kinda a full time job when there's a tournament haha