Don't kill his stacks just reduce the numbers to fewer units.
If you can , get some demonesses to swap one of his necros close to your units, unfortunately he has no bone dragons that would fly fast down to your troops so you could had swaped the b.dragons with the necros.
Necro call first the dead units , try to necro call his necros ... after they die.
If you can't necro call the position some unit over the dead stack.
Also i think that elven archers do more damage than the cyclope do , and use them to kill the crystal. I don't find dragons usable in that fight , only thing they could do is give you some mana .. think about replacing them with something else.
You could also use royal/warrior/ the other one.. thorns and sprout thorns on a dead stack.
Good luck
edit : there was a thread somewhere in the forum about this fight , search for it , it might help you.