Here's how I beat that part.
As soon as the level begins, summon Wyvern. Never use the firebreath ability, it puts you in too much danger. Spam Fire Sphere or whatever it's called, the first ability that just shoots a regular fireball. Every time it cools down fire it again. On hard difficulty for me the ballista towers fell in 2 hits. If you have any health loss activate the healing spell every time it cools down. Once you clear the first set of ballista towers, feel free to continue far far ahead of the zeppelin taking down everything in your path. Whenever it cools down, use the patriarch ability on a cluster of 3+ towers to wipe them all out. Never use the ability that does damage in a radius around you, you have to be too close and it puts you in danger. Always be moving, I was constantly holding the "strafe sideways" button to fly in a circle as I spammed my first ability.
I died the first time I tried the level, but the second time, using what I described above, I completed it with ease.
