Dance of Death - "[Campaign 9 - Mission 2]"

How to defeat the spectral dragon in mission 2?

I only defeated the dragon on the second try, after realizing that I first needed to defeat the liches and not use enchanted units.
So, I focused on defeating the liches first, and then I started attacking the dragon.
He has a tricky ability – he casts a spell on units, and instead of dealing damage to him, they increase his health. A purple glow appears over such units, and when you hover the cursor over the dragon, it doesn't show the damage that will be dealt.