it has been a while but ok
either you mass offensive spells to kill em
or use units like cyclopses(high resistance )
emerald dragon (invisibility and slow kill them with spells)
with a undead army(ghost+necromancers)
kill a dragon stack then ressurect it with the necromancers
1 big stack of skeleton archers with dragon arrows(they pwn dragons)
like said above use any kind of dragons