Zeppelin Escort Mission on Nightmare Skill

I used a combination of 1 Fireball plus Firebreath, and that seemed to work for me, even on Nightmare skill. (Even with the short cooldown on Fireball, I didn't like waiting for it.)

1. Fly as far ahead of the zeppelin as you can.
2. Kill all the towers.
3. Don't worry about the flying enemies any more than you need to stay alive.