I don't know if your actual character build influences what you get as a dragon but in dragon form my fire sphere ability was 15 and took so little mana that it was regenerated by the time it was ready for use again. Fire breath on the other hand was practically worthless
. Used all of my magic within 2 seconds and took longer to breath something to death than it did to simply hit it twice with fire spheres.
So, assuming you get the same build regardless of your character, use nothing but fire spehere during that last mission and go from island to island in a straight line. The zepplin's going to be moving pretty much straight through the blockade of enemies (not the smartest zepplin pilot ever -_-) so minimize damage to it by killing everything in it's path long before it gets there. None of the enemies will attack the zepplin from behind nor will they fly ahead to attack the it so as long as you clear the area before the zepplin gets there it should be safe. Using only fire sphere may seem to be slower than a combination of skills but, again assuming the build is the same regardless of character, I got the Zepplin through with more than 70% of it's health remaining and it only began taking damage around the end when it approached the some half a dozen+ islands of ballistas, nests and lightning towers clustered tightly together. Plus fire sphere has a blast radius so it generally kills several flyers in one hit as well (even indirectly when attacking the nests.)
Spoiler
If we're being honest I lost it the first time and had to replay it a second but I was a bit disappointed with the forced crashing of the zepplin because when the pilot actually started to say "we can't make it back, we're too damage" he still had 90% of his health -_-.
