giants very bad against dragons and their specials doesnt work on dragons at all.
try get alot of melee units and have spell dragon slayer on them. dont stack up your units in rows so they can shoot at 2 the same time.
i would suggest slow sturdy melee units in gangs for those battles. the time when u can destroy all enemys with same 5 mobs are simply over.
i played through game as mage and at that point i used same tactic as u with nukes and archers. at the points when u meet fast units and they can fly all over before u can nuke destroy that tactic.
get some units with high initative and try slow the dragons if u still wanna use archer mobs.
a way to get a super good mob with 7 initative is to clean gremlin castle otherwise they are not avaible until end of elven lands at lvl 23-25.
i think i used giants (5),cyklops(6)cannoners,tough slow dwarf fighters lvl 3 many,and orc shamans.
i used the orc shamans untill endgame when i couldnt replace losses and they didnt sell em more in orc castle.
rage skill drain souls is good agaisnt dragons , kills of 20-30% of their total hp in a stack. very very usefull when u start meeting dragons in stacks of 50-100 hehe