Please offer some advice.

The Disintegrate spell kills one type of unit. But it does so decisively. If you have Magic Dragons in your army, you have a chance to kill his hero with a simple fireball. And the hero himself is unlikely to risk killing a dragon with magic, because the magic mirror will work :) And Disintegrate will only be able to kill 4 or 5 dragons, but by that time you will have a good number of dragons :D If he has Black Dragons, summon Phoenixes and cast Dragon Strength on them, etc.