I try not use so much air units because it's seem to me as a "cheat", in normal mode i use mass air units but in hard i want to challenge me and win with land units mix with air troops.
With that said, i know that maybe will be impossible to win "Last Stand of Golden Guard", without mass dragons hitkilling the castle of dwarf units and Merlon castle
I will try that in Roars of the Forest, i use the old strategy in this kind of mission of using only heroes to attack enemy units to not lose my units until the crucial point of mission like boss fight, base invade.
One mission that i will try to win with land units is "End of a Uprising", i will try not use trebuchets except the inner base to defeat grey.